drm/i915/fbc: Introduce intel_fbc_set_false_color()

Pull the direct FBC register frobbing out from the debugfs code
into the fbc code. Also add a vfunc for this so we don't need
extra platforms checks.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211104144520.22605-11-ville.syrjala@linux.intel.com


Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>