dma-buf: Remove internal lock

The internal dma-buf lock isn't needed anymore because the updated
locking specification claims that dma-buf reservation must be locked
by importers, and thus, the internal data is already protected by the
reservation lock. Remove the obsoleted internal lock.

Acked-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>