mesa: fix validate for secondary interpolator

mesa: fix validate for secondary interpolator

This patch fixes multiple problems:
- the interpolator check was duplicated
- both had arg instead of argRep
- I split it into color and alpha for better readability and error msg
- the DOT4 check only applies to color instruction according to the spec
- made the DOT4 check fatal, and improved the error msg

Piglit: spec/ati_fragment_shader/error08-secondary

v2: fixed formatting, added spec quotations

Signed-off-by: Miklós Máté <mtmkls@gmail.com>