drm/amd/display: Remove redundant null checks

drm/amd/display: Remove redundant null checks

The null checks are redundant as they were already dereferenced
previously, as reported by Coverity; therefore the null checks
are removed.

This fixes 7 REVERSE_INULL issues reported by Coverity.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>