drm/i915: Eliminate obj->state usage in g4x/vlv/chv wm computation

Use explicit old/new states instead of relying on obj->state.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170823152226.22938-4-ville.syrjala@linux.intel.com