drm/i915/skl: Don't allow disabling ppgtt and execlists on gen9+

Running the driver without execlists and hence PPGTT (either aliasing or
full) isn't a supported configuration on gen9+.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>