media: vivid: use vzalloc for dev->bitmap_out

When vivid is unloaded it used vfree to free dev->bitmap_out,
but it was actually allocated using kmalloc. Use vzalloc
instead, conform what vivid-vid-cap.c does.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>