drm/i915/guc: Stop using mutex while sending CTB messages

We are no longer using descriptor to hold G2H replies and we are
protecting access to the descriptor and command buffer by the
separate spinlock, so we can stop using mutex.

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/20210603051630.2635-17-matthew.brost@intel.com