drm/amd/display: Enable output_bpc property on all outputs

[Why]
Connector property output_bpc is available on DP/eDP only. New IGT tests
would benifit if this property works on HDMI.

[How]
Enable this read-only property on all types of connectors.

Signed-off-by: Stylon Wang <stylon.wang@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org