drm/amd/display: add null check for invalid opps

[Why]
In cases where number of pipes available is less
 than num_opp, there will opp instances that are
 null

[How]
Add null check to skip over these opp instances

Fixes: 40de8403 ("drm/amd/display: Update OPP counter from new interface")
Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Acked-by: Roman Li <roman.li@amd.com>
Signed-off-by: Samson Tam <samson.tam@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>