drm/i915/xehp: Enable ccs/dual-ctx in RCU_MODE

We have to specify in the Render Control Unit Mode register
when CCS is enabled.

v2:
 - Move RCU_MODE programming to a helper function.  (Tvrtko)
 - Clean up and clarify comments.  (Tvrtko)
 - Add RCU_MODE to the GuC save/restore list.  (Daniele)
v3:
 - Move this patch before the GuC ADS update to enable compute engines;
   the definition of RCU_MODE and its insertion into the save/restore
   list moves to this patch.  (Daniele)
v4:
 - Call xehp_enable_ccs_engines() directly in guc_resume() and
   execlists_resume() rather than adding an extra layer of wrapping to
   the engine->resume() vfunc.  (Umesh)

Bspec: 46034
Original-author: Michel Thierry
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Cc: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Aravind Iddamsetty <aravind.iddamsetty@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220302001554.1836066-1-matthew.d.roper@intel.com