drm/i915: Expand subslice mask

Currently, the subslice_mask runtime parameter is stored as an
array of subslices per slice. Expand the subslice mask array to
better match what is presented to userspace through the
I915_QUERY_TOPOLOGY_INFO ioctl. The index into this array is
then calculated:
  slice * subslice stride + subslice index / 8

v2: Fix 32-bit build
v3: Use new helper function in SSEU workaround warning message
v4: Use GEM_BUG_ON to force developers to use valid SSEU configurations
    per platform (Chris)

Signed-off-by: Stuart Summers <stuart.summers@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190823160307.180813-12-stuart.summers@intel.com