Prep work for a following patch, that introduces a cache to map from
program state (all shader stages) plus variant key to pre-baked hw
state (which could be emit'd via CP_SET_DRAW_STATE, for example).
To do that, we really want the variant key to be immutable, and to
treat the binning pass shader as an extra shader stage, rather than
as a VS variant.

Signed-off-by: Rob Clark <robdclark@gmail.com>