drm/amd/display: Add connector debugfs for "output_bpc"

[Why]
This will be useful for verifying whether we enter the correct output
color depth from IGT.

[How]
Locks the connector and associated CRTC if available and outputs
the current and maximum output bpc values.

Example:

cat /sys/kernel/debug/dri/0/DP-1/output_bpc
Current: 8
Maximum: 10

v2: Drop unneeded connector status check

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: David Francis <David.Francis@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>