drm/i915/guc: fix comment about fallback to execlists

We stopped supporting fallback to execlists in commit 121981fa
(drm/i915/guc: Combine enable_guc_loading|submission modparams). We
do instead reset and retry in some cases, depending on the workarounds
required by the platform.

Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20181022230427.5616-4-daniele.ceraolospurio@intel.com