drm/1915/fbc: Replace plane->has_fbc with a pointer to the fbc instance

With multiple fbc instances we need to find the right one for each
plane. Rather than going looking for the right instance every time
let's just replace the has_fbc boolean with a pointer that gets us
there straight away.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211104144520.22605-18-ville.syrjala@linux.intel.com
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>