drm/amd/display: fixed divide by zero kernel crash during dsc enablement

[why]
During dsc enable, a divide by zero condition triggered the
kernel crash.

[how]
An IGT test, which enable the DSC, was crashing at the time of
restore the default dsc status, becaue of h_totals value
becoming 0. So add a check before divide condition. If h_total
is zero, gracefully ignore and set the default value.

kernel panic log:

	[  128.758827] divide error: 0000 [#1] PREEMPT SMP NOPTI
	[  128.762714] CPU: 5 PID: 4562 Comm: amd_dp_dsc Tainted: G        W         5.4.19-android-x86_64 #1
	[  128.769728] Hardware name: ADVANCED MICRO DEVICES, INC. Mauna/Mauna, BIOS WMN0B13N Nov 11 2020
	[  128.777695] RIP: 0010:hubp2_vready_at_or_After_vsync+0x37/0x7a [amdgpu]
	[  128.785707] Code: 80 02 00 00 48 89 f3 48 8b 7f 08 b ......
	[  128.805696] RSP: 0018:ffffad8f82d43628 EFLAGS: 00010246
	......
	[  128.857707] CR2: 00007106d8465000 CR3: 0000000426530000 CR4: 0000000000140ee0
	[  128.865695] Call Trace:
	[  128.869712] hubp3_setup+0x1f/0x7f [amdgpu]
	[  128.873705] dcn20_update_dchubp_dpp+0xc8/0x54a [amdgpu]
	[  128.877706] dcn20_program_front_end_for_ctx+0x31d/0x463 [amdgpu]
	[  128.885706] dc_commit_state+0x3d2/0x658 [amdgpu]
	[  128.889707] amdgpu_dm_atomic_commit_tail+0x4b3/0x1e7c [amdgpu]
	[  128.897699] ? dm_read_reg_func+0x41/0xb5 [amdgpu]
	[  128.901707] ? dm_read_reg_func+0x41/0xb5 [amdgpu]
	[  128.905706] ? __is_insn_slot_addr+0x43/0x48
	[  128.909706] ? fill_plane_buffer_attributes+0x29e/0x3dc [amdgpu]
	[  128.917705] ? dm_plane_helper_prepare_fb+0x255/0x284 [amdgpu]
	[  128.921700] ? usleep_range+0x7c/0x7c
	[  128.925705] ? preempt_count_sub+0xf/0x18
	[  128.929706] ? _raw_spin_unlock_irq+0x13/0x24
	[  128.933732] ? __wait_for_common+0x11e/0x18f
	[  128.937705] ? _raw_spin_unlock_irq+0x13/0x24
	[  128.941706] ? __wait_for_common+0x11e/0x18f
	[  128.945705] commit_tail+0x8b/0xd2 [drm_kms_helper]
	[  128.949707] drm_atomic_helper_commit+0xd8/0xf5 [drm_kms_helper]
	[  128.957706] amdgpu_dm_atomic_commit+0x337/0x360 [amdgpu]
	[  128.961705] ? drm_atomic_check_only+0x543/0x68d [drm]
	[  128.969705] ? drm_atomic_set_property+0x760/0x7af [drm]
	[  128.973704] ? drm_mode_atomic_ioctl+0x6f3/0x85a [drm]
	[  128.977705] drm_mode_atomic_ioctl+0x6f3/0x85a [drm]
	[  128.985705] ? drm_atomic_set_property+0x7af/0x7af [drm]
	[  128.989706] drm_ioctl_kernel+0x82/0xda [drm]
	[  128.993706] drm_ioctl+0x225/0x319 [drm]
	[  128.997707] ? drm_atomic_set_property+0x7af/0x7af [drm]
	[  129.001706] ? preempt_count_sub+0xf/0x18
	[  129.005713] amdgpu_drm_ioctl+0x4b/0x76 [amdgpu]
	[  129.009705] vfs_ioctl+0x1d/0x2a
	[  129.013705] do_vfs_ioctl+0x419/0x43d
	[  129.017707] ksys_ioctl+0x52/0x71
	[  129.021707] __x64_sys_ioctl+0x16/0x19
	[  129.025706] do_syscall_64+0x78/0x85
	[  129.029705] entry_SYSCALL_64_after_hwframe+0x44/0xa9
Signed-off-by: Robin Singh <robin.singh@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Reviewed-by: Robin Singh <Robin.Singh@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn20/dcn20_hubp.c

 /*
- * Copyright 2012-17 Advanced Micro Devices, Inc.
+ * Copyright 2012-2021 Advanced Micro Devices, Inc.
  *
  * Permission is hereby granted, free of charge, to any person obtaining a
  * copy of this software and associated documentation files (the "Software"),
@@ -181,11 +181,14 @@ void hubp2_vready_at_or_After_vsync(struct hubp *hubp,
 	else
 		Set HUBP_VREADY_AT_OR_AFTER_VSYNC = 0
 	*/
+	if (pipe_dest->htotal != 0) {
 		if ((pipe_dest->vstartup_start - (pipe_dest->vready_offset+pipe_dest->vupdate_width
 			+ pipe_dest->vupdate_offset) / pipe_dest->htotal) <= pipe_dest->vblank_end) {
 			value = 1;
 		} else
 			value = 0;
+	}
+
 	REG_UPDATE(DCHUBP_CNTL, HUBP_VREADY_AT_OR_AFTER_VSYNC, value);
 }