drm/i915: Skip useless watermark/FIFO related work on VLV/CHV when not needed

Check whether anything relevant has actually change when we compute new
watermarks for each plane in the state. If the watermarks for no
primary/sprite planes changed we don't have to recompute the FIFO split
or reprogram the DSBARB registers. And even the cursor watermarks didn't
change we can skip the merge+invert step between all the planes on
the pipe as well.

v2: s/noninverted/raw/ for consistency with other platforms
v3: Drop duplicated vlv_get_fifo_size() call during init

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/20170302171508.1666-9-ville.syrjala@linux.intel.com