softpipe: Enable PIPE_CAP_MIXED_COLORBUFFER_FORMATS

softpipe: Enable PIPE_CAP_MIXED_COLORBUFFER_FORMATS
  
It seems softpipe actually supports this. This change enables the
following piglits as passing without regressions in the gpu test set:

 gl-3.1-mixed-int-float-fbo
 gl-3.1-mixed-int-float-fbo int_second
 fbo-blending-format-quirks

Changes for deqp:

 dEQP-GLES2.functional.fbo.completeness.attachment_combinations.rbo_tex_none_none QualityWarning -> Pass
 dEQP-GLES2.functional.fbo.completeness.attachment_combinations.rbo_tex_none_rbo QualityWarning -> Pass
 dEQP-GLES2.functional.fbo.completeness.attachment_combinations.rbo_tex_none_tex QualityWarning -> Pass
 dEQP-GLES2.functional.fbo.completeness.attachment_combinations.rbo_tex_rbo_none QualityWarning -> Pass
 dEQP-GLES2.functional.fbo.completeness.attachment_combinations.rbo_tex_tex_none QualityWarning -> Pass
 dEQP-GLES2.functional.fbo.completeness.attachment_combinations.tex_rbo_none_none QualityWarning -> Pass
 dEQP-GLES2.functional.fbo.completeness.attachment_combinations.tex_rbo_none_rbo QualityWarning -> Pass
 dEQP-GLES2.functional.fbo.completeness.attachment_combinations.tex_rbo_none_tex QualityWarning -> Pass
 dEQP-GLES2.functional.fbo.completeness.attachment_combinations.tex_rbo_rbo_none QualityWarning -> Pass
 dEQP-GLES2.functional.fbo.completeness.attachment_combinations.tex_rbo_tex_none QualityWarning -> Pass

 dEQP-GLES3.functional.fbo.completeness.samples.rbo0_rbo0_tex Fail -> Pass
 dEQP-GLES3.functional.fbo.completeness.samples.rbo0_tex_none Fail -> Pass
 dEQP-GLES3.functional.fbo.completeness.samples.rbo1_rbo1_rbo1 Fail -> Pass
 dEQP-GLES3.functional.fragment_out.random.* NotSupported -> Pass

 dEQP-GLES31.functional.shaders.builtin_functions.common.frexp.*_fragment Fail -> Pass
 dEQP-GLES31.functional.shaders.builtin_functions.common.frexp.*_vertex Fail -> Pass
 dEQP-GLES31.functional.shaders.builtin_functions.precision.frexp.*_fragment.* Fail -> Pass
 dEQP-GLES31.functional.shaders.builtin_functions.precision.frexp.*_vertex.* Fail -> Pass

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>