drm/i915/guc: limit forcewake to blitter domain in guc_send

drm/i915/guc: limit forcewake to blitter domain in guc_send

The forcewake_get call in the guc_send_mmio function was added to
avoid getting and releasing forcewake on each register access.
While this makes sense, all GuC registers are in the blitter range
so no need to wake all the wells.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1490366919-34715-1-git-send-email-daniele.ceraolospurio@intel.com
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>