[media] v4l: ti-vpe: register video device only when firmware is loaded

vpe fops(vpe_open in particular) should be called only when VPDMA firmware
is loaded. File operations on the video device are possible the moment it is
registered.

Currently, we register the video device for VPE at driver probe, after calling
a vpdma helper to initialize VPDMA and load firmware. This function is
non-blocking(it calls request_firmware_nowait()), and doesn't ensure that the
firmware is actually loaded when it returns.

We remove the device registration from vpe probe, and move it to a callback
provided by the vpe driver to the vpdma library, through vpdma_create().

The ready field in vpdma_data is no longer needed since we always have firmware
loaded before the device is registered.

A minor problem with this approach is that if the video_register_device
fails(which doesn't really happen), the vpe platform device would be registered.
however, there won't be any v4l2 device corresponding to it.

Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>