drm/vmwgfx: avoid calling vzalloc with a 0 size in vmw_get_cap_3d_ioctl()

In vmw_get_cap_3d_ioctl(), a user can supply 0 for a size that is
used in vzalloc(). This eventually calls dump_stack() (in warn_alloc()),
which can leak useful addresses to dmesg.

Add check to avoid a size of 0.

Cc: <stable@vger.kernel.org>
Signed-off-by: Murray McAllister <murray.mcallister@insomniasec.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>