drm/i915/guc: Prepare to handle messages from CT RECV buffer

GuC can respond to our commands not only by updating SEND buffer
descriptor, but can also send a response message over RECV buffer.
Guc can also send unsolicited request messages over RECV buffer.
Let's start reading those messages and make placeholders
for actual response/request handlers.

v2: misc improvements (Michal)
v3: change response detection (Michal)
    invalid status is protocol error (Michal)
v4: rebase
v5: fix checkpatch (Michel)
    don't use fields before check (Jani)
    add some documentation (Michal)

Signed-off-by: Michal Wajdeczko <[email protected]>
Cc: Oscar Mateo <[email protected]>
Cc: Michel Thierry <[email protected]>
Cc: Daniele Ceraolo Spurio <[email protected]>
Reviewed-by: Michel Thierry <[email protected]> # 4.5
Cc: Jani Nikula <[email protected]>
Signed-off-by: Chris Wilson <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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