Full GPU reset causes GuC to be reset. This means that every time we're
doing a reset, we need to talk to GuC and tell it about doorbells.
Let's separate the communication part (create_doorbell) from our
internal bookkeeping (reserve_doorbell) so that we can cleanly separate
the initialization done at module load from reinitialization done at
reset in the following patch.
While I'm here, let's also add a proper (although slightly asymetric)
cleanup that doesn't try to communicate with GuC after it's already
gone, getting rid of "expected" warnings caused by GuC action failures
on module unload.

Note that I've also removed one of the tests (bitmap out of sync), since
it doesn't make much sense anymore - bitmaps are now not expected to
change during the lifetime of a client.
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171213221352.7173-5-michal.winiarski@intel.com