drm/i915: Add intel_atomic_add_affected_planes()

drm_atomic_add_affected_planes() only considers planes which
are logically enabled in the uapi state. For bigjoiner we need
to consider planes logically enabled in the hw state. Add a
helper for that.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201124201156.17095-2-ville.syrjala@linux.intel.com


Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>