i965/gs(gen7.5): Set max_gs_threads correctly
WIP: make the state atom for compiling Gen7 geometry shaders.  TODO: maybe disentangle
WIP: generalize DEBUG_VS for GS
WIP: turn on GS, and mark GS stuff as untested for safety
!UPSTREAM: Generalize some error messages for GS.

TODO: find more places to generalize.
TODO: Don't use "VS/GS" terminology.  Use "vec4" terminology, since that will endure longer.
i965/gs(gen7): Allocate URB space for use by geometry shader
!UPSTREAM: i965/gs(gen7): Allocate push constant space
i965/vs: generalize push const upload in preparation for GS
i965/gs: set up GS state