svga: Unify gmr and mob surface pool managers

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.

This patch reduces the number of buffer managers by unifying similar managers
and performing refactors where necessary.

Signed-off-by: Maaz Mombasawala <mombasawalam@vmware.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>
Reviewed-by: Zack Rusin <zackr@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25832>