[media] media: blackfin: bfin_capture: use vb2_fop_mmap/poll

No need to reinvent the wheel. Just use the already existing
functions provided by vb2.

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Tested-by: Scott Jiang <scott.jiang.linux@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>