drm/i915: Don't use staged config in intel_mst_pre_enable_dp()

For the conversion to atomic. The pre_enable() hooks are called as part
of the crtc enable sequence, at which point the staged config was
already made effective. Furthermore, the function actually changes
hardware state, so it should anyway deal with current and not staged
config.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>