drm/i915/display: Use drm helper instead of own loop for damage clips

drm/i915/display: Use drm helper instead of own loop for damage clips

Use existing drm_atomic_helper_damage_merged from generic drm code
instead of implementing own loop to iterate over damage_clips.

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220823112920.352563-4-jouni.hogander@intel.com
Reviewed-by: Mika Kahola <mika.kahola@intel.com>