driver: use mesa's MIN2/MAX2 instead of our own

Trivial code style change.
driver: null handling on binding

No longer segfault when binding NULL shader or sampler view.
driver: add PIPE_SHADER_CAP_TGSI_(POW|LRP)_SUPPORTED

We need lowering for these instructions as they're not in the
ISA. As Mesa can do this for us, expose them as shader capabilities.