winsys/sw/dri: don't use GNU void pointer arithmetic

winsys/sw/dri: don't use GNU void pointer arithmetic

Resolves build issues like the following:

src/gallium/winsys/sw/dri/dri_sw_winsys.c:203:31: error: pointer of type ‘void *’ used in arithmetic [-Werror=pointer-arith]
        data = dri_sw_dt->data + (dri_sw_dt->stride * box->y) + box->x * blsize;
                               ^
src/gallium/winsys/sw/dri/dri_sw_winsys.c:203:62: error: pointer of type ‘void *’ used in arithmetic [-Werror=pointer-arith]
        data = dri_sw_dt->data + (dri_sw_dt->stride * box->y) + box->x * blsize;
                                                              ^

Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>