[Why]

A null pointer deference can occur if crtc is null in
amdgpu_dm_crtc_handle_crc_irq. This can happen if get_crtc_by_otg_inst
returns NULL during dm_crtc_high_irq, leading to a hang in some IGT
test cases.

[How]

Check that CRTC is non-null before accessing its fields.
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Sun peng Li <Sunpeng.Li@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>