drm/i915/guc: Kill USES_GUC_SUBMISSION macro

use intel_uc_uses_guc_submission() directly instead, to be consistent in
the way we check what we want to do with the GuC.

v2: do not go through ctx->vm->gt, use i915->gt instead

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com> #v1
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200218223327.11058-3-daniele.ceraolospurio@intel.com