drm/i915: Fix regression in the sprite plane update split

7e4bf45dbd99a965c7b5d5944c6dc4246f171eb5 introduced the regression.
We fix it by doing the right assignment of crtc_y

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=83747


Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>