drm/i915/guc: Always copy CT message to new allocation

Since most of future CT traffic will be based on G2H requests,
instead of copying incoming CT message to static buffer and then
create new allocation for such request, always copy incoming CT
message to new allocation. Also by doing it while reading CT
header, we can safely fallback if that atomic allocation fails.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Cc: Piotr Piórkowski <piotr.piorkowski@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210603051630.2635-19-matthew.brost@intel.com