drm/i915/skl: Add SKL HW status to SSEU status

Add a new section to the 'i915_sseu_status' debugfs entry to
report the currently enabled counts of slice, subslice, and
execution units on the device. The count of enabled subslice
per slice represents the most enabled subslice on any one
slice for devices where imbalances may exist. Similarly, the
count of enabled EU per subslice represents the most enabled
EU on any one subslice.

Collect this device status for Skylake by reading the Gen9
power gate control ack message registers. Power gate control
operates on EU in pairs, therefore our reported counts of
enabled EU can be overestimated by one for each pair in which
one EU is fused-off.

Signed-off-by: Jeff McGee <jeff.mcgee@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>