drm/virtio: Use struct_size() helper in kmalloc()

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes.

This code was detected with the help of Coccinelle and, audited and
fixed manually.

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20200617215707.GA16785@embeddedor


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>