drm/i915/uc: Add explicit DISABLED state for firmware

We really need to have separate NOT_SUPPORTED state (for
lack of hardware support) and DISABLED state (to indicate
user decision) as we will have to take special steps even
if GuC firmware is now disabled but hardware exists and
could have been previously used.

v2: fix logic (Chris/CI)
v3: use proper check to avoid probe failure (CI)
v4: explain status transitions (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/20190816205658.15020-1-michal.wajdeczko@intel.com