drm/exynos: DRIVER_BUS_PLATFORM is not a driver feature

DRIVER_BUS_PLATFORM is a bus type used internally in the DRM core, not a
flag for the drm_driver::driver_features field.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>