gallium: add st_api feature mask to prevent advertising MS visuals

v2: use a define for the maximum sample count
v3: also test odd sample counts (r300 supports MS3)

While multisample renderbuffers are supported by mesa, MS visuals
are not, so we need a way to tell dri/st not to advertise them even
if the gallium driver does support multisampled surfaces.

Otherwise applications selecting these non-functional visuals would
run into trouble ...

Reviewed-by: Brian Paul <[email protected]>

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