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

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

Instead of returning small data in response status dword,
GuC may append longer data as response message payload.
If caller provides response buffer, we will copy received
data and use number of received data dwords as new success
return value. We will WARN if response from GuC does not
match caller expectation.

v2: fix timeout and checkpatch warnings (Michal)
v3: fix checkpatch again (Michel)
    update wait function name (Michal)
    no need for spinlock_irqsave (MichalWi)
    no magic numbers (MichalWi)
    must check before use (Jani)
    add some more documentation (Michal)
v4: update documentation (Michal)

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> #2.5
Cc: Michal Winiarski <michal.winiarski@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180327121439.70096-1-michal.wajdeczko@intel.com