drm/i915: Rename bunch of vlv_ watermark structures to g4x_

We'll be wanting to share some of these watermark structures on g4x,
so let's rename them to have a g4x_ prefix instead of vlv_.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170421181432.15216-5-ville.syrjala@linux.intel.com


Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>