i965/fs: Implement SIMD16 dual source blending.

i965/fs: Implement SIMD16 dual source blending.

From the SNB PRM, volume 4, part 1, page 193:

"The dual source render target messages only have SIMD8 forms due to
 maximum message length limitations. SIMD16 pixel shaders must send two of
 these messages to cover all of the pixels. Each message contains two colors
 (4 channels each) for each pixel in the message payload."

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=82831
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>