drm/i915: Enable SAGV support for Gen12

Flip the switch and enable SAGV support
for Gen12 also.

Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200514074853.9508-4-stanislav.lisovskiy@intel.com