drm/i915: fix subslice mask array size

We have a subslice mask per slice, not per subslice.
MAX_SUBSLICES > MAX_SLICES, so the wrong size didn't cause any issue
apart from using extra memory.

Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181106182918.5748-1-daniele.ceraolospurio@intel.com