drm/etnaviv: Don't set drm_device->platformdev

It's deprecated and only should be used by drivers which still use
drm_platform_init, not by anyone else.

And indeed it's entirely unused and can be nuked.

Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Christian Gmeiner <christian.gmeiner@gmail.com>
Acked-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471034937-651-6-git-send-email-daniel.vetter@ffwll.ch