drm/i915/xehp: handle fused off CCS engines

HW resources are divided across the active CCS engines at the compute
slice level, with each CCS having priority on one of the cslices.
If a compute slice has no enabled DSS, its paired compute engine is not
usable in full parallel execution because the other ones already fully
saturate the HW, so consider it fused off.

v2 (José):
 - moved it to its own function
 - fixed definition of ccs_mask

v3 (Matt):
 - Replace fls() condition with a simple IP version test

v4 (Matt):
 - Don't try to calculate a ccs_mask using
   intel_slicemask_from_dssmask() until we've determined that we're
   running on an Xe_HP platform where the logic makes sense (and won't
   overflow).

Cc: Stuart Summers <stuart.summers@intel.com>
Cc: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Cc: Ashutosh Dixit <ashutosh.dixit@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Stuart Summers <stuart.summers@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220302052008.1884985-1-matthew.d.roper@intel.com