drm/vmwgfx: Ditch the vmw_dummy_query_bo_prepare function

Combine it with vmw_dummy_query_bo_create, and also make sure
we use tryreserve when reserving the bo to avoid any lockdep warnings
We are sure the tryreserve will always succeed since we are
the only users at that point.
In addition, allow the vmw_bo_pin function to pin/unpin system memory.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>