drm/i915: Fix SKL cursor watermarks

drm/i915: Fix SKL cursor watermarks

Use intel_wm_plane_visible() to determine cursor visibility for SKL+
also. Previously SKL+ would check the actual visibility which now
conflicts with the assumptions in intel_legacy_cursor_update().

We also change SKL+ to compute the cursor watermarks based on the
unclipped cursor size, just as we do on all the other platforms.
Using the clipped size could now result in garbage results.

Testcase: igt/kms_chv_cursor_fail
Fixes: ad68989fc980 ("drm/i915: Fix legacy cursor vs. watermarks for ILK-BDW")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100195
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170314151050.12194-2-ville.syrjala@linux.intel.com
Tested-by: Dorota Czaplejewicz <dorota.czaplejewicz@collabora.co.uk>
Tested-by: Jari Tahvanainen <jari.tahvanainen@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>