drm/amd/display: Remove the redundant NULL check

drm/amd/display: Remove the redundant NULL check

Static analysis shows that pointer "timing" cannot be NULL because it
points to the object "struct dc_crtc_timing".

Remove the extra NULL check. It is meaningless and harms the readability
of the code.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Andrey Vatoropin <a.vatoropin@crpt.ru>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>