drm/core: use helper to check driver features

The patch replaces direct access to driver_features field
by calls to helper function.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>