drm/i915/fbc: Register per-crtc debugfs files

drm/i915/fbc: Register per-crtc debugfs files

Expose FBC debugfs files for each crtc. These may or may not point
to the same FBC instance depending on the platform.

We leave the old global debugfs files in place until
igt catches up to the new per-crtc approach.

v2: Take a trip via intel_crtc_debugfs_add() (Jani)

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211213151435.9700-1-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>