staging/mmal-vchiq: Fix incorrect static vchiq_instance.

For some reason lost in history function vchiq_mmal_init used
a static variable for storing the vchiq_instance.
This value is retrieved from vchiq per instance, so worked fine
until you try to call vchiq_mmal_init multiple times concurrently
when things then go wrong. This seemed to happen quite frequently
if using the cutdown firmware (no MMAL or VCSM services running)
as the vchiq_connect then failed, and one or other vchiq_shutdown
was working on an invalid handle.

Remove the static so that each caller gets a unique vchiq_instance.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>