drm/i915/guc: Support programming the EU priority in the GuC descriptor

In GuC submission mode the EU priority must be updated by the GuC rather
than the driver as the GuC owns the programming of the context descriptor.

Given that the GuC code uses the GuC priorities, we can't use a generic
function using i915 priorities for both execlists and GuC submission.
The existing function has therefore been pushed to the execlists
back-end while a new one has been added for GuC.

v2: correctly use the GuC prio.

Cc: John Harrison <john.c.harrison@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Aravind Iddamsetty <aravind.iddamsetty@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220504234636.2119794-1-daniele.ceraolospurio@intel.com