drm/i915: Pass crtc state to vlv_compute_wm_level()

Rather than accessing crtc->config in vlv_compute_wm_level() let's
pass in the crtc state explicitly. One step closer to atomic.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1480354637-14209-16-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>