drm/i915/display: Prepare DRRS for frontbuffer rendering drop

drm/i915/display: Prepare DRRS for frontbuffer rendering drop

Frontbuffer rendering will be dropped for modern platforms but
before that we to prepare DRRS for it.

intel_drrs_flush and intel_drrs_invalidate will not be called
for platforms that will not support frontbuffer rendering so DRRS
needs another way to be notified about to page flips so it can change
between high and low refresh rates as needed.

Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210903221036.34770-3-jose.souza@intel.com