vc04_services: codec: Allocate the max number of buffers on the VPU

The VPU's API can't match the use of VIDIOC_CREATE_BUFS to add buffers
to the internal pool whilst a port is enabled, therefore allocate
the maximum number of buffers possible in V4L2 to avoid the issue.
As these are only buffer headers, the overhead is relatively small.

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