drm/i915/fbc: Move FBC debugfs stuff into intel_fbc.c

In order to encapsulate FBC harder let's just move the debugfs
stuff into intel_fbc.c.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211124113652.22090-12-ville.syrjala@linux.intel.com


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