drm/i915: adjust get_crtc_fence_y_offset() to use base.y instead of crtc.y

This is to use clipped y coordinate here. I left get_crtc_fence_y_offset()
function itself in place as oneliner to maintain comment above it why this
is done.

Signed-off-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1508270891-22186-6-git-send-email-juhapekka.heikkila@gmail.com
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 835e2d2..f01c800 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1116,6 +1116,8 @@ struct intel_fbc {
 			 */
 			int adjusted_x;
 			int adjusted_y;
+
+			int y;
 		} plane;
 
 		struct {