drm/i915: Eliminate usage of pipe_wm_parameters from ILK-style WM (v2)

Just pull the info out of the CRTC state structure rather than staging
it in an additional structure.

Note that we use cstate->active rather than intel_crtc->active which may
appear to be a change in behavior.  However since we're no longer trying
to recalculate watermarks during the "pipe off" stage of a modeset,
intel_crtc->active and cstate->active should always be identical when
watermarks are calculated (at least for ILK-style platforms).

v2: Clarify reasoning for cstate->active and add a WARN_ON to the code
    to assert that it really is always identical to intel_crtc->active
    as expected.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>