drm/vmwgfx: Fix overlay when using Screen Targets

drm/vmwgfx: Fix overlay when using Screen Targets

This code was never updated to support Screen Targets.
Fixes a bug where Xv playback displays a green screen instead of actual
video contents when 3D acceleration is disabled in the guest.

Fixes: c8261a961ece ("vmwgfx: Major KMS refactoring / cleanup in preparation of screen targets")
Reported-by: Doug Brown <doug@schmorgal.com>
Closes: https://lore.kernel.org/all/bd9cb3c7-90e8-435d-bc28-0e38fee58977@schmorgal.com
Signed-off-by: Ian Forbes <ian.forbes@broadcom.com>
Tested-by: Doug Brown <doug@schmorgal.com>
Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240719163627.20888-1-ian.forbes@broadcom.com