drm/i915/skl: Provide a Skylake version of get_plane_config()

Universal planes have changed a bit the register organization.

v2: Rebase on top of the latest drm-intel-nightly

v3: Use PLANE_SIZE to retrieve the fb size (Tvrtko)
    Don't use BUG() (Tvrtko)

v4: Use MISSING_CASE (Daniel)

Reviewed-By: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>