drm/i915/guc: Disable automatic GuC firmware loading

New GuC code is logging errors at runtime suspend and resume which
causes CI testing to log "orange" status. Default to not trying to
load the firmware until this is resolved.

Example of the log:

 [drm] RC6 on
 [drm:intel_runtime_suspend] Suspending device
 [drm:host2guc_action [i915]] *ERROR* GUC: host2guc action 0x501 failed. ret=-110 status=0x00000501 response=0x40000000
 ...
 [drm:intel_runtime_resume] Resuming device
 [drm:host2guc_action [i915]] *ERROR* GUC: host2guc action 0x502 failed. ret=-110 status=0x00000502 response=0x40000000
 [drm:intel_runtime_resume] Device resumed

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Dave Gordon <david.s.gordon@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Chris Harris <chris.harris@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1464017675-12257-1-git-send-email-tvrtko.ursulin@linux.intel.com