drm/etnaviv: allow build with COMPILE_TEST

Make it possible to compile test the driver on other platforms.

Cc: l.stach@pengutronix.de
Cc: linux+etnaviv@armlinux.org.uk
Cc: christian.gmeiner@gmail.com
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170126225621.12314-7-noralf@tronnes.org