drm/i915: Add plane vfuncs to enable/disable flip_done interrupt

Prepare for more platforms with async flip support by turning
the flip_done interrupt enable/disable into plane vfuncs.

Cc: Karthik B S <karthik.b.s@intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210111163711.12913-6-ville.syrjala@linux.intel.com


Reviewed-by: Karthik B S <karthik.b.s@intel.com>