iris: Initial commit of a new 'iris' driver for Intel Gen8+ GPUs.

iris: Initial commit of a new 'iris' driver for Intel Gen8+ GPUs.

This commit introduces a new Gallium driver for Intel Gen8+ GPUs,
named 'iris_dri.so' after the hardware.

Developed by:
- Kenneth Graunke (overall driver)
- Dave Airlie (shaders, conditional render, overflow query, Gen8 port)
- Chris Wilson (fencing, pinned memory, ...)
- Jordan Justen (compute shaders)
- Jason Ekstrand (image load store)
- Caio Marcelo de Oliveira Filho (tessellation control passthrough)
- Rafael Antognolli (auxiliary buffer fixes)
- The rest of the i965 contributors and the Mesa community