drm/i915/guc: Use correct lock for accessing guc->mmio_msg

Guc->mmio_msg is set under the guc->irq_lock in guc_get_mmio_msg so it
should be consumed under the same lock from guc_handle_mmio_msg.

I am not sure if the overall flow here makes complete sense but at least
the correct lock is now used.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20201120095636.1987395-1-tvrtko.ursulin@linux.intel.com