[media] media: blackfin: bfin_capture: drop bcap_get_unmapped_area()

this patch drops bcap_get_unmapped_area() and uses
vb2_fop_get_unmapped_area() helper provided by the vb2 core.

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