drm/i915: Only use update_wm_{pre,post} for pre-ilk platforms

Now that vlv/chv have more proper wm programming support, let's reduce
the the update_wm_{pre,post} flags to only cover the pre-ilk platforms.
When we finally convert those as well we can drop these flags entirely.

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/20170302171508.1666-12-ville.syrjala@linux.intel.com