drm/i915/guc: Enable send function only after successful init

drm/i915/guc: Enable send function only after successful init

It is safer to setup valid send function after successful GuC
hardware initialization. In addition we prepare placeholder
where we can setup any alternate GuC communication mechanism.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170502103243.54940-1-michal.wajdeczko@intel.com
[ickle: Fixup ENODEV for an impossible error path]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>