drm/vmwgfx/vmwgfx_validation: Use flexible-array member instead of zero-length array

Suppresses the following coccinelle warning:

drivers/gpu/drm/vmwgfx/vmwgfx_validation.c:85:15-22: WARNING use flexible-array member instead

Signed-off-by: Zou Wei <zou_wei@huawei.com>
Signed-off-by: Zack Rusin <zackr@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210323130204.45439-1-zou_wei@huawei.com