vmwgfx: Remove screen object active list

It isn't used for anything. Replace with an active bool.

Also make a couple of functions return void instead of int
since their return value wasn't checked anyway.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jakbo Bornecrantz <jakob@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>