[media] media: mx3_camera: buf_init() add buffer state check

This patch checks the state of the buffer when calling .buf_init() method.
This is needed for the USERPTR buffer type, because in that case
.buf_init() is called every time a buffer is queued, and not only once
during the preparation stage, like in the MMAP case. Without this check
buffers get initialised repeatedly, which also leads to the allocation
of new DMA descriptors, of which there is only a final relatively small
number available. Both MMAP and USERPTR methods were successfully tested.

Signed-off-by: Alex Gershgorin <alexg@meprolight.com>
[g.liakhovetski@gmx.de: remove mx3_camera_buffer::state completely]
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>