drm/i915/guc: Assert we have the doorbell before setting it up

As our early doorbell is split between early allocation and a late setup
after we have a channel to the GuC, it may happen due to a lapse of
programmer judgement that we try to setup an invalid doorbell. Make use
of our has_doorbell() function to check the doorbell does exist for the
client before we try and tell the guc about it. In doing so, we prevent
the compiler from warning about the otherwise unused function in some
configurations.

Reported-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180501075203.12458-1-chris@chris-wilson.co.uk