drm/vc4: Fix X/Y positioning of planes using T_TILES modifier

X/Y positioning of T-format buffers is quite tricky and the current
implementation was failing to position a plane using this format
correctly when the CRTC X, Y or both X and Y offsets were negative.
It was also failing when the SRC X/Y offsets were != 0.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180803092231.26446-5-boris.brezillon@bootlin.com