drm/vmwgfx: Add an interface to pin a resource v3

For screen targets it appears we need to pin surfaces while they are bound
as screen targets, so add a small interface to do that.

v2: Always increase pin_count on pin.
v3: Add missing reservation sem.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>