drm/i915: Schedule DRRS work from intel_drrs_enable()

Schedule the DRRS downclock work already from intel_drrs_enable()
instead of waiting around for a frontbuffer flush that may or
may not ever come.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220315132752.11849-7-ville.syrjala@linux.intel.com


Reviewed-by: José Roberto de Souza <jose.souza@intel.com>