drm/fb helper: don't call drm_crtc_helper_set_config

Go through the interface vtable instead, because not everyone might be
using the crtc helper code.

Cc: dri-devel@lists.freedesktop.org
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>