genxml: Make BLEND_STATE command support variable length array.

genxml: Make BLEND_STATE command support variable length array.

We need to emit BLEND_STATE, which size is 1 + 2 * nr_draw_buffers
dwords (on gen8+), but the BLEND_STATE struct length is always 17. By
marking it size 1, which is actually the size of the struct minus the
BLEND_STATE_ENTRY's, we can emit a BLEND_STATE of variable number of
entries.

For gen6 and gen7 we set length to 0, since it only contains
BLEND_STATE_ENTRY's, and no other data.

With this change, we also change the code for blorp and anv to emit only
the needed BLEND_STATE_ENTRY's, instead of always emitting 16 dwords on
gen6-7 and 17 dwords on gen8+.

v2:
   - Use designated initializers on blorp and remove 0 from
   initialization (Jason)
   - Default entries to disabled on Vulkan (Jason)
   - Rebase code.

Signed-off-by: Rafael Antognolli <rafael.antognolli@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>