drm/fb helper: don't call drm_helper_connector_dpms directly

Yet again a case where the fb helper is too intimate with the crtc
helper and calls a crtc helepr function directly instead of going
through the interface vtable.

This fixes console blanking in drm/i915 with the new i915-specific
modeset code.

Reported-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Tested-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>