drm/i915: Fix watermarks for VLV/CHV

drm/i915: Fix watermarks for VLV/CHV

commit 92826fcdfc14 ("drm/i915: Calculate watermark related members in the crtc_state, v4.")
broke thigns by removing the pre vs. post wm update distinction. We also
lost the pre plane wm update entirely for VLV/CHV from the crtc enable
path.

This caused underruns on modeset and plane enable/disable on CHV,
and often those can lead to a dead pipe.

So let's bring back the pre vs. post thing, and let's toss in an
explicit wm update to valleyview_crtc_enable() to avoid having to
put it into the common code.

This is more or less a partial revert of the offending commit.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: drm-intel-fixes@lists.freedesktop.org
Fixes: 92826fcdfc14 ("drm/i915: Calculate watermark related members in the crtc_state, v4.")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1457543247-13987-4-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>