drm/i915/guc: Refactor the retrieval of guc_process_desc

Move the common "client->vaddr + client->proc_desc_offset" to its own
function, __get_process_desc() to match the newly established pattern.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170323230000.20786-1-chris@chris-wilson.co.uk


Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>