drm/i915/bxt: Support BXT in SSEU device status dump

Modify the Gen9 SSEU device status logic to support Broxton.
Broxton reuses the Skylake power gate acknowledgment registers but
has at most 1 slice and 3 subslices. Broxton supports subslice
power gating within its single slice.

Signed-off-by: Jeff McGee <jeff.mcgee@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>