drm/amd/display: Fix check for null function ptr

[Why]
Bug fix for null function ptr (should check for NULL instead of not
NULL)

[How]
Fix if condition

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Samson Tam <samson.tam@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>