drm/i915/guc: Don't allow GuC submission

Due to the upcoming changes to the GuC ABI interface, we must
disable GuC submission mode until final ABI will be available
on all GuC firmwares.

To avoid regressions on systems configured to run with no longer
supported configuration "enable_guc=3" or "enable_guc=1" clear
GuC submission bit.

v2: force switch to non-GuC submission mode
v3: use GEM_BUG_ON (Joonas)

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: John Spotswood <john.a.spotswood@intel.com>
Cc: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Cc: Tony Ye <tony.ye@intel.com>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Cc: Jeff Mcgee <jeff.mcgee@intel.com>
Cc: Antonio Argenziano <antonio.argenziano@intel.com>
Cc: Sujaritha Sundaresan <sujaritha.sundaresan@intel.com>
Cc: Martin Peres <martin.peres@linux.intel.com>
Acked-by: Martin Peres <martin.peres@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190527183613.17076-3-michal.wajdeczko@intel.com