gallium: Make TGSI_SEMANTIC_FOG register four-component wide.

D3D9 Shader Model 2 restricted the fog register to one component,
http://msdn.microsoft.com/en-us/library/windows/desktop/bb172945.aspx ,
but that restriction no longer exists in Shader Model 3, and several
WHCK tests enforce that.

So this change:
- lifts the single-component restriction TGSI_SEMANTIC_FOG
  from Gallium interface
- updates the Mesa state tracker to enforce output fog has (f, 0, 0, 1)
- draw module was updated to leave TGSI_SEMANTIC_FOG output registers
  alone

Several gallium drivers that are going out of their way to clear
TGSI_SEMANTIC_FOG components could be simplified in the future.

Thanks to Si Chen and Michal Krol for identifying the problem.

Testing done: piglit fogcoord-*.vpfp tests

Reviewed-by: Roland Scheidegger <[email protected]>

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