drm/i915/display/debug: Expose crtc current bpc via debugfs

This new debugfs will expose the currently using bpc by crtc.
It is very useful for verifying whether we enter the correct
output color depth from IGT.

This patch will also add the connector's max supported bpc to
"i915_display_info" debugfs.

Example:
cat /sys/kernel/debug/dri/0/crtc-0/i915_current_bpc
Current: 8

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Bhanuprakash Modem <bhanuprakash.modem@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220519095149.3560034-3-bhanuprakash.modem@intel.com