drm/vc4: Fix TILE_Y_OFFSET definitions

Y_OFFSET field starts at bit 8 not 7.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180803092231.26446-1-boris.brezillon@bootlin.com