drm/i915/guc: Use guc_class instead of engine_class in fw interface

GuC has its own defines for the engine classes. They're currently
mapping 1:1 to the defines used by the driver, but there is no guarantee
this will continue in the future. Given that we've been caught off-guard
in the past by similar divergences, we can prepare for the changes by
introducing helper functions to convert from engine class to GuC class and
back again.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210603051630.2635-21-matthew.brost@intel.com