media: vivid: move the detection part out of vivid_create_instance

The vivid_create_instance() function is too harry. Smatch can't
process it, because it takes too long.

The detection part is an important piece of it. As such, there
are even comments before and after such block.

So, it makes sense to just move it to a separate function.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>