drm/i915: Calculate watermark configuration during atomic check (v2)

v2: Don't forget to actually check the cstate->active value when
    tallying up the number of active CRTC's.  (Ander)

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Smoke-tested-by: Paulo Zanoni <przanoni@gmail.com>
Link: http://patchwork.freedesktop.org/patch/59561/