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.
driver: implement SIN and COS instructions

This commit implements the TGSI SIN and COS instructions.

There is still an unhandled case on cubox, which has a gc600
without trigonometric instructions support. For this GPU
we will need to fall back on a Taylor expansion.
GC880 - replay/cube_etna2 - draw interleaved vertices
Add support for rendering using unified memmory on GC880 (similar to GC2000).