svga: Unify gmr and mob surface pool managers

The svga driver contains several buffer managers with gmr* and mob* prefixes but they are of similar functionality. The underlying vmwgfx kernel driver allocates memory of one type - mobs if supported by the virtual hardware, and gmrs otherwise - and returns an opaque pointer to userspace, so several managers are reduntant.

There is also no need to have a separate shareable pool of surfaces, we can make all surfaces shareable.