drm/i915: Calculate vlv/chv intermediate watermarks correctly, v3.

The watermarks it should calculate against are the old optimal watermarks.
The currently active crtc watermarks are pure fiction, and are invalid in
case of a nonblocking modeset, page flip enabling/disabling planes or any
other reason.

When the crtc is disabled or during a modeset the intermediate watermarks
don't need to be programmed separately, and could be directly assigned
to the optimal watermarks.

CXSR must always be disabled in the intermediate case for modesets, else
we get a WARN for vblank wait timeout.

Also rename crtc_state to new_crtc_state, to distinguish it from the old state.

Changes since v1:
- Use intel_atomic_get_old_crtc_state. (ville)
Changes since v2:
- Always unset cxsr during modeset.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171115163157.14372-1-maarten.lankhorst@linux.intel.com
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>