anv: Disable dual source blending when shader doesn't support it on gen8+

anv: Disable dual source blending when shader doesn't support it on gen8+

Dual source blending behaviour is undefined when shader doesn't
have second color output.

 "If SRC1 is included in a src/dst blend factor and
  a DualSource RT Write message is not used, results
  are UNDEFINED. (This reflects the same restriction in DX APIs,
  where undefined results are produced if “o1” is not written
  by a PS – there are no default values defined)."

Dismissing fragment in such situation leads to a hang on gen8+
if depth test in enabled.

Since blending cannot be gracefully fixed in such case and the result
is undefined - blending is simply disabled.

v2 (Jason Ekstrand):
 - Apply the workaround to each individual entry
 - Emit a warning through debug_report

Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>