mesa: fix texstore with GL_COLOR_INDEX data

mesa: fix texstore with GL_COLOR_INDEX data

This got broken by 3dbf5bf6571e0c9d3e4febce01dea82be190d9d2.
GL_COLOR_INDEX data is still supported (in legacy contexts), but the new
texstore_swizzle path cannot handle it (and didn't detect this).
Unfortunately there's no piglit test trying to specify textures with a
GL_COLOR_INDEX source format, and I don't really understand how all the color
map stuff which is used by this works, but this caused conform failures
(with a reported mesa implementation error when trying to figure out the color
mapping).

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>