drm: Provide a driver hook for drm_dev_release()

Some state is coupled into the device lifetime outside of the
load/unload timeframe and requires teardown during final unreference
from drm_dev_release(). For example, dmabufs hold both a device and
module reference and may live longer than expected (i.e. the current
pattern of the driver tearing down its state and then releasing a
reference to the drm device) and yet touch driver private state when
destroyed.

v2: Export drm_dev_fini() and move the responsibility for finalizing the
drm_device and freeing it to the release callback. (If no callback is
provided, the core will call drm_dev_fini() and kfree(dev) as before.)
v3: Remember to add drm_dev_fini() to drm_drv.h
v4: Tidy language for kerneldoc
v5: Cross reference from drm_dev_init() to note that driver->release()
allows for arbitrary embedding.
v6: Refer to driver data rather than driver state, as state is now
becoming associated with the struct drm_atomic_state and friends.

Signed-off-by: Chris Wils...