media: videobuf2: remove unused variable

Commit 2cc1802f ("media: vb2: Keep dma-buf buffers mapped until
they are freed") removed code leaving a local variable unused.

Remove it to avoid a compiler warning.

Fixes: 2cc1802f ("media: vb2: Keep dma-buf buffers mapped until they are freed")

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>