drm/bochs: add drm_driver.release callback.

Call bochs_unload via drm_driver.release to make sure we release stuff
when it is safe to do so.  Use drm_dev_{enter,exit,unplug} to avoid
touching hardware after device removal.  Tidy up here and there.

v4: add changelog.
v3: use drm_dev_*().
v2: move hardware deinit to pci_remove().

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20200211135218.22871-1-kraxel@redhat.com