drm/etnaviv: Use drm_dev_unref, not drm_put_dev

drm/etnaviv: Use drm_dev_unref, not drm_put_dev

drm_put_dev is the old midlayer-broken device cleanup function, but
etnaviv has a proper unbind function which first unregisters and then
drops the final reference. No functional change since
drm_dev_unregister happens to be idempotent.

Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Russell King <linux+etnaviv@armlinux.org.uk>
Cc: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161208110739.24417-1-daniel.vetter@ffwll.ch