i965: Add a driconf option to disable GL_ARB_blend_func_extended.

Unigine Heaven (at least) has a bug where it incorrectly uses the
GL_ARB_blend_func_extended extension.

Dual source blending allows two color outputs per render target;
individual shader outputs can be assigned to be either the first or
second blending input by setting the 'index' via one of two methods:

- An API call: glBindFragDataLocationIndexed()
- The GLSL 'layout' qualifier provided by GL_ARB_explicit_attrib_location

Both of these only work on user defined fragment shader outputs; it's an
error to use either on built-in outputs like gl_FragData.

Unigine uses gl_FragData and gl_FragColor exclusively, and doesn't even
attempt to use either method to set index == 1.  However, it does set
the blending function to SRC1 enums, which requires a fragment shader
output with index == 1 or else rendering is undefined.

In other words, enabling ARB_blend_func_extended causes Unigine to
render incorrectly, resulting in an apparent regression, even though our
driver code (as far as I can tell) is perfectly fine.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=50291
Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Chad Versace <[email protected]>
Reviewed-by: Eric Anholt <[email protected]>

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