drm/i915/guc: Implement response handling in send_mmio()

We're using data encoded in the status MMIO as return value from send
function, but GuC may also write more data in remaining MMIO regs.
Let's copy content of these registers to the buffer provided by caller.

v2: new line (Michel)
v3: updated commit message

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Oscar Mateo <oscar.mateo@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/20180326194829.58836-5-michal.wajdeczko@intel.com