media: venus: use contig vb2 ops

This driver uses the SG vb2 ops, but effectively only ever accesses the
first entry of the SG table, indicating that it expects a flat layout.
Switch it to use the contiguous ops to make sure this expected invariant
is always enforced. Since the device is supposed to be behind an IOMMU
this should have little to none practical consequences beyond making the
driver not rely on a particular behavior of the SG implementation.

Reported-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>