10-bit output is incomplete on GXM, G12A, G12B, SM1

[chewitt]

GXBB has an 8-bit output pipeline. GXL has an internal 10-bit pipeline but only supports 8-bit output. GXM and later support a 10-bit internal pipeline and output.

Support for 10-bit output in the current GXL/GXM focussed hardware decoders is incomplete, so everything outputs 8-bit.