drm/i915: Enable eDP DRRS on ilk/snb port A

Nothing special about ivb+ here, if DRRS works on ivb+ port A
it should work just as well on ilk/snb. So let's enable
that.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220311172428.14685-15-ville.syrjala@linux.intel.com