[media] videobuf-dma-contig: restore buffer mapping for uncached bufers

from commit a8f3c203
restore the mapping scheme for uncached buffers,
which was changed in a common scheme for cached and uncached.
This apparently was wrong, and was probably intended only for cached buffers.
the fix fixes the crash observed while mapping uncached buffers.

Signed-off-by: Lad, Prabhakar <prabhakar.lad@ti.com>
Signed-off-by: Hadli, Manjunath <manjunath.hadli@ti.com>
Acked-by: Federico Vaga <federico.vaga@gmail.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Cc: stable@kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>