gpu: ipu-v3: don't depend on DRM being enabled

The PRE/PRG drivers, which need the DRM infrastructure, are only used
from the output path, so we skip building them into the ipu-v3 driver
if CONFIG_DRM is not enabled.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>