drm/i915/guc: Use dedicated flag to track submission mode

Instead of relying on enable_guc modparam to represent actual
GuC submission mode, use dedicated flag and look at modparam
only to check if submission was explicitly disabled by the user.

v2: rebased, simplified condition (Chris)

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
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/20190731223321.36436-4-michal.wajdeczko@intel.com