drm/i915/guc: init engine directly in GuC submission mode

Instead of starting the engine in execlists submission mode and then
switching to GuC, start directly in GuC submission mode. The initial
setup functions have been copied over from the execlists code
and simplified by removing the execlists submission-specific parts.

v2: remove unneeded unexpected starting state check (Chris)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: John Harrison <john.c.harrison@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20210113021236.8164-4-daniele.ceraolospurio@intel.com