drm/i915/fbc: Pimp the FBC debugfs output

Now that each plane tracks its own no_fbc_reason we can print that
out in debugfs, and we can also show which plane is currently
selected for FBC duty.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211124113652.22090-21-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>