DRM: Armada: prime refcounting bug fix

Commit 011c2282 changed the way refcounting on imported dma_bufs
works, and this hadn't been spotted while forward-porting Armada.
Reflect the changes in that commit into the Armada driver.

Reviewed-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>