drm/i915/skl: Add Gen9 LRC size

The LRC increased in size on gen9. Make sure we return the right
size in get_lr_context_size()

v2. Corrected the size, should be 22 pages. I unintentionally mailed out
a test patch w/ size equaling 23 pages.

Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Michael H. Nguyen <michael.h.nguyen@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>