drm/i915/guc: Start protecting access to CTB descriptors

We want to stop using guc.send_mutex while sending CTB messages
so we have to start protecting access to CTB send descriptor.

For completeness protect also CTB receive descriptor.

Add spinlock to struct intel_guc_ct_buffer and start using it.

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>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210603051630.2635-15-matthew.brost@intel.com