drm/vmwgfx: Implement dma_fence_ops properly

vmwgfx's fencing predates dma_fence and as a result dma_fence_ops was never
properly implemented, especially with respect to enabling signaling.

Because of this dma_fence callbacks don't work properly. This change
implements enable_signaling properly so that dma_fence callbacks now
work as expected.

It also removes vmwgfx's custom implementation of fence callbacks
and removes vmwgfx's custom dma_fence_ops::wait function which is no
longer necessary now that enable_signaling works.

Signed-off-by: Ian Forbes <ian.forbes@broadcom.com>
Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://lore.kernel.org/r/20250530183510.733175-2-ian.forbes@broadcom.com