drm/vmwgfx: Optimize the buffer object swap_notify callback somewhat.

Only try to unmap cached maps when the buffer is moved into or out from
vram. Otherwise the underlying pages stay the same.

Also when unbinding resources from MOBs about to move, make sure we're
really moving out of MOB memory.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Deepak Rawat <drawat@vmware.com>