i965: Add tessellation evaluation shaders

i965: Add tessellation evaluation shaders

The TES is essentially a post-tessellator VS, which has access to the
entire TCS output patch, and a special gl_TessCoord input.  Otherwise,
they're very straightforward.

This patch implements SIMD8 tessellation evaluation shaders for Gen8+.
The tessellator can generate a lot of geometry, so operating in SIMD8
mode (8 vertices per thread) is more efficient than SIMD4x2 mode (only
2 vertices per thread).  I have another patch which implements SIMD4x2
mode for older hardware (or via an environment variable override).

We currently handle all inputs via the pull model.

v2: Improve comments (suggested by Jordan Justen).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>