drm/i915/guc: rename __create/destroy_doorbell

The 2 functions don't create or destroy anything, they just update the
doorbell state in memory. Use init and fini instead for clarity.

Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20181022230427.5616-1-daniele.ceraolospurio@intel.com