genxml: Make gen6-7 blending look more like gen8

genxml: Make gen6-7 blending look more like gen8

This renames BLEND_STATE to BLEND_STATE_ENTRY and adds an new struct
BLEND_STATE which is just an array of 8 BLEND_STATE_ENTRYs.  This will make
it much easier to write gen-agnostic blend handling code.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: "12.0" <mesa-stable@lists.freedesktop.org>
(cherry picked from commit aaa202ebe7b86f71f596bf01f6177f4cb66f2c4c)