panfrost: Set alpha_zero_nop/one_store flags
Some Mali GPUs can avoid storing to the tilebuffer if src alpha = 0, and can replace blending with a store if src alpha = 1. This saves power in the common case of alpha blending. Add predicates to check if these optimizations are valid for a given blend equation.
Then set the appropriate flags in the panfrost drivers.
I tested this in the GL driver. panvk is compile tested only.