drm/i915/uc: Stop talking with GuC when resetting

Knowing that GuC will be reset soon, we may stop all communication
immediately without doing graceful cleanup as it is not needed.

This patch will also help us capture any unwanted/unexpected attempts
to talk with GuC after we decided to reset it. And we need to keep
'disable' part as current and upcoming firmware still expect graceful
cleanup.

v2: update commit msg

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
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/20190523172555.2780-1-michal.wajdeczko@intel.com