vc1dec: Invoke edge emulation regardless of MV precision for 1-MV chroma

This is required due to the way VC-1 handles chroma pull-back which may end
up causing negative chroma MV for zero luma MV. Edge emulation needs to be
invoked in such cases.

This only affects vertical component of chroma motion vector.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>