drm/etnaviv: fix reference leak when mmaping imported buffer

drm_gem_prime_mmap() takes a reference on the GEM object, but before that
drm_gem_mmap_obj() already takes a reference, which will be leaked as only
one reference is dropped when the mapping is closed. Drop the extra
reference when dma_buf_mmap() succeeds.

Cc: stable@vger.kernel.org
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>