drm/amd/display: Remove redundant checks for opp

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

This fixes 2 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>