drm/i915: WaEnableForceRestoreInCtxtDescForVCS is for video engines only

Also check for correct revision id in each Gen9 platform (SKL until B0
and BXT until A0).

Cc: Nick Hoath <nicholas.hoath@intel.com>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Arun Siluvery <arun.siluvery@linux.intel.com>
Tested-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>