media: vim2m: allow setting the default transaction time via parameter

While there's a control to allow setting it at runtime, as the
control handler is per file handler, only the application setting
the m2m device can change it. As this is a custom control, it is
unlikely that existing apps would be able to set it.

Due to that, and due to the fact that v4l2-mem2mem serializes all
accesses to a m2m device, trying to setup two GStreamer
v4l2videoconvert instance at the same time will cause frame drops.

So, add an alternate way of setting its default via a modprobe parameter.

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