drm/vmwgfx: Convert screen targets to new helpers v3

Also implements the missing readback function and
fixes page flip in case of no event.

v2:
- Adapt to the work done for screen targets for 2d, in particular
Handle proxy surface updates.
- Remove execbuf quirks since we now use fifo reserve / commit.
- Revert the initial placement of vmw dma buffers.

v3: Address review comments.

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