i965g: switch to SIMD8 mode
i965g: disable stream output for gen7
i965g: reserve 15 GRFs on gen7 for MRFs

The maximum message length is 15.
i965g: clean up VUE write
i965g: GEN7 has fixed-function SOL stage
i965g: unify gen6 and gen7 depth buffer emission

Plus, use BRW_SURFACE_2D for PIPE_TEXTURE_CUBE.