drm/xe: Fix UBSAN shift-out-of-bounds failure

Here is the failure stack:
[   12.988209] ------------[ cut here ]------------
[   12.988216] UBSAN: shift-out-of-bounds in ./include/linux/log2.h:57:13
[   12.988232] shift exponent 64 is too large for 64-bit type 'long unsigned int'
[   12.988235] CPU: 4 PID: 1310 Comm: gnome-shell Tainted: G     U             6.9.0-rc6+prerelease1158+ #19
[   12.988237] Hardware name: Intel Corporation Raptor Lake Client Platform/RPL-S ADP-S DDR5 UDIMM CRB, BIOS RPLSFWI1.R00.3301.A02.2208050712 08/05/2022
[   12.988239] Call Trace:
[   12.988240]  <TASK>
[   12.988242]  dump_stack_lvl+0xd7/0xf0
[   12.988248]  dump_stack+0x10/0x20
[   12.988250]  ubsan_epilogue+0x9/0x40
[   12.988253]  __ubsan_handle_shift_out_of_bounds+0x10e/0x170
[   12.988260]  dma_resv_reserve_fences.cold+0x2b/0x48
[   12.988262]  ? ww_mutex_lock_interruptible+0x3c/0x110
[   12.988267]  drm_exec_prepare_obj+0x45/0x60 [drm_exec]
[   12.988271]  ? vm_bind_ioctl_ops_execute+0x5b/0x740 [xe]
[   12.988345]  vm_bind_ioctl_ops_execute+0x78/0x740 [xe]

It is caused by the value 0 of parameter num_fences in function
drm_exec_prepare_obj.  And lead to in function __rounddown_pow_of_two,
"0 - 1" causes the shift-out-of-bounds.

By design drm_exec_prepare_obj() should be called only when there are
fences to be reserved. If num_fences is 0, calling drm_exec_lock_obj()
is sufficient as was done in commit 9377de4c ("drm/xe/vm: Avoid
reserving zero fences")

Cc: Nirmoy Das <nirmoy.das@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Shuicheng Lin <shuicheng.lin@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://lore.kernel.org/all/24d4a9a9-c622-4f56-8672-21f4c6785476@amd.com
Link: https://patchwork.freedesktop.org/patch/msgid/20240507130411.630361-1-shuicheng.lin@intel.comSigned-off-by: Lucas De Marchi <lucas.demarchi@intel.com>

drivers/gpu/drm/xe/xe_vm.c

@@ -2692,7 +2692,7 @@ static int vma_lock_and_validate(struct drm_exec *exec, struct xe_vma *vma,
 
 	if (bo) {
 		if (!bo->vm)
-			err = drm_exec_prepare_obj(exec, &bo->ttm.base, 0);
+			err = drm_exec_lock_obj(exec, &bo->ttm.base);
 		if (!err && validate)
 			err = xe_bo_validate(bo, xe_vma_vm(vma), true);
 	}
@@ -2777,7 +2777,7 @@ static int vm_bind_ioctl_ops_lock_and_prep(struct drm_exec *exec,
 	struct xe_vma_op *op;
 	int err;
 
-	err = drm_exec_prepare_obj(exec, xe_vm_obj(vm), 0);
+	err = drm_exec_lock_obj(exec, xe_vm_obj(vm));
 	if (err)
 		return err;