drm/etnaviv: Expose our reservation object when exporting a dmabuf.

Without this, polling on the dma-buf (and presumably other devices
synchronizing against our rendering) would return immediately, even
while the BO was busy.

Signed-off-by: Eric Anholt <eric@anholt.net>
Cc: stable@vger.kernel.org
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Russell King <linux+etnaviv@armlinux.org.uk>
Cc: Christian Gmeiner <christian.gmeiner@gmail.com>
Cc: etnaviv@lists.freedesktop.org
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>