drm/debug: Expose connector's max supported bpc via debugfs

It's useful to know the connector's max supported bpc for IGT
testing. Expose it via a debugfs file on the connector "output_bpc".

Example: cat /sys/kernel/debug/dri/0/DP-1/output_bpc

V2:
* Fix typo in comments (Harry)

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Bhanuprakash Modem <bhanuprakash.modem@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220519095149.3560034-2-bhanuprakash.modem@intel.com