[media] videobuf2: Do not unconditionally map S/G buffers into kernel space

The one in-tree videobuf2-dma-sg driver (mmp-camera) has no need for a
kernel-space mapping of the buffers; one suspects that most other drivers
would not either.  The videobuf2-dma-sg module does the right thing if
buf->vaddr == NULL - it maps the buffer on demand if somebody needs it.  So
let's not map the buffer at allocation time; that will save a little CPU
time and a lot of address space in the vmalloc range.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>