drm/gud: Free buffers on device removal

Free transfer and compression buffers on device removal instead of at
DRM device removal time. This ensures that the usual 2x8MB buffers are
released when the device is unplugged and not kept around should
userspace keep the DRM device fd open.

At least Ubuntu 20.04 doesn't release the DRM device on unplug.

The damage_lock mutex is not destroyed because it is used outside the
drm_dev_enter/exit block in gud_pipe_update(). AFAICT it's possible for
an open fbdev descriptor to trigger a commit after the USB device is gone.

v2: Don't destroy damage_lock

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210701170748.58009-1-noralf@tronnes.org