drm/i915/guc: A little bit more of doorbell sanitization

drm/i915/guc: A little bit more of doorbell sanitization

Some recent refactoring patches have left the doorbell creation outside
the GuC client allocation, which does not make a lot of sense (a client
without a doorbell is something useless). Move it back there, and
refactor the init_doorbell_hw consequently.

Thanks to this, we can do some other improvements, like hoisting the
check for GuC submission enabled out of the enable function.

v2: Rebased.

Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>