drm/amd/display: Add null pointer guards where needed

[Why]
Some functions whose output is typically checked for null are not being
checked for null at several call sites, causing some static analysis
tools to throw an error.

[How]
Add null pointer guards around functions that typically have them at
other call sites.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Sung Lee <sung.lee@amd.com>
Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Josip Pavic <josip.pavic@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>