wayland: Don't rely on static variable for identifying wl_drm buffers

Now that libEGL has been fixed to not leak all kinds of symbols, gbm
links to its own copy of the libwayland-drm.a helper library.  That means
we can't rely on comparing the addresses of a static vtable symbol in that
library to determine if a wl_buffer is a wl_drm_buffer.  Instead, we
move the vtable into the wl_drm struct and use that for comparing.

Backported from 360a141f24a9d00891665b7fedb77ffb116944ca.

https://bugs.freedesktop.org/show_bug.cgi?id=69437

Cc: 9.2 <mesa-stable@lists.freedesktop.org>