drm/vc4: Fix negative X/Y positioning on SAND planes

Commit 3e407417 ("drm/vc4: Fix X/Y positioning of planes using
T_TILES modifier") fixed the problem with T_TILES format, but left
things in a non-working state for SAND formats. Address that now.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20181207083606.15449-1-boris.brezillon@bootlin.com