drm/i915: Compute VLV/CHV FIFO sizes based on the PM2 watermarks

Let's compute the watermarks first and the FIFO size second. This way we
can make sure the FIFO split is the most accommodating to the watermarks.
Previously we could have potentially computed a FIFO split that couldn't
accommodate the PM2 watermarks simply due to a bad split even if the
total FIFO size would have been sufficient.

It'll also allow us to avoid recomputing the wms for all planes whenever
the FIFO split would change. Thus we don't have to add any extra planes
to the state when the FIFO needs to be repartitioned.

To help with this we'll keep around copies of the non-inverted
watermarks in the crtc state. For now that doesn't help too much, but
once we start to do the watermark computation only for the planes
that change we'll need the non-inverted values around for the other
planes.

v2: s/noninverted/raw/ for consistency with other platforms
    Fix the memset() of the "raw" watermarks

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170302171508.1666-7-ville.syrjala@linux.intel.com