media: vivid: move the devnode creation logic to a separate function

In order to reduce even further the size of the big
vivid_create_instance() function, let's place the part of the
logic which creates the device nodes into a separate function.

    With this and the past patches, those warnings finally
    vanishes:

            drivers/media/test-drivers/vivid/vivid-core.c: drivers/media/test-drivers/vivid/vivid-core.c:1189 vivid_create_instance() parse error: turning off implications after 60 seconds
            drivers/media/test-drivers/vivid/vivid-core.c: drivers/media/test-drivers/vivid/vivid-core.c:1257 vivid_create_instance() parse error: __split_smt: function too hairy.  Giving up after 303 seconds

The init code also seems more organized after breaking the long
function into a smaller set.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>