intel: Serialize drmPrimeFDToHandle with struct_mutex

It is possible to hit a race condition in create_from_prime, when trying
to import a BO that's currently being freed. In case of prime sharing
we'll succesfully get a handle, but fail on get_tiling call, potentially
confusing the caller (and requiring different locking scheme than with
sharing using flink). Wrap fd_to_handle with struct_mutex to force
a more consistent behaviour between prime/flink, convert fprintf to DBG
when handling errors.

(From Chris:
  The race is that the kernel returns us the same file-private handle as
  the first thread, but that first thread is about to call gem_close
  (thereby removing the handle from the file completely) and does so
  between us acquiring the handle and taking the mutex. If we take
  the mutex, then we acquire the refcnt on the bo prior to the first
  thread completing its unref (and so preventing the early close). Or we
  acquire the handle after the earlier close, in which case we are the new
  owner.
)

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Testcase: igt/drm_import_export/import-close-race-prime
Signed-off-by: Rafał Sapała <rafal.a.sapala@intel.com>
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>