drm/i915: Calculate pipe watermarks into CRTC state (v3)

A future patch will calculate these during the atomic 'check' phase
rather than at WM programming time, so let's store the watermark
values we're planning to use in the CRTC state; the values actually
active on the hardware remains in intel_crtc.

While we're at it, do some minor restructuring to keep ILK and SKL
values in a union.

v2: Don't move cxsr_allowed to state (Maarten)

v3: Only calculate watermarks in state.  Still keep active watermarks in
    intel_crtc itself.  (Ville)

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>