i965: Fix alpha to one with dual color blending.

i965: Fix alpha to one with dual color blending.

The BLEND_STATE documentation says that alpha to one must be disabled
when dual color blending is enabled.  However, it appears that it simply
fails to override src1 alpha to one.

We can work around this by leaving alpha to one enabled, but overriding
SRC1_ALPHA to ONE and ONE_MINUS_SRC1_ALPHA to ZERO.  This appears to be
what the other driver does, and it looks like it works despite the
documentation saying not to do it.

Fixes spec/ext_framebuffer_multisample/alpha-to-one-dual-src-blend *
Piglit tests.

v2: Add UNUSED to shut up warning on generations which don't use this.

Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>