drm/vc4: use platform_register_drivers

Use platform_register_drivers instead of open coding the iteration over
component platform drivers in the vc4_drv module.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170317170059.17821-1-p.zabel@pengutronix.de