anv: merge tessellation's primitive mode in merge_tess_info()

anv: merge tessellation's primitive mode in merge_tess_info()

SPIR-V tessellation shaders that were created from HLSL will have
the primitive generation domain set in tessellation control shader
(hull shader in HLSL) instead of the tessellation evaluation shader.

v2:
- Add assert (Kenneth)

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>