mesa: fix out of bounds stack access on big endian

The texture format code relies on a python-generated atlas of structs that describe a lookup table for texture swizzling. Many of these texture formats contain the index "6" used for this lookup. The 6th index just so happens to represent a "don't care" value, however the out of bounds read is still best to be avoided. The address sanitizer finds this issue pretty immediately but it only shows up on big endian because the textures don't need this on little.