drm/i915/guc: Prepare send() function to accept bigger response

This is a preparation step for the upcoming patches.
We already can return some small data decoded from the command
status, but we will need more in the future.

v2: add explicit response buf size
v3: squash with helper patch

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Oscar Mateo <oscar.mateo@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@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/20180326194829.58836-4-michal.wajdeczko@intel.com