gallium/util: fix util_color_[un]pack[-ub] to be strict aliasing safe

gallium/util: fix util_color_[un]pack[-ub] to be strict aliasing safe

use pointer to union instead of void pointer.
gcc complained a lot, depending what the pointer originally actually was.
Looks like it's in fact maybe legal to cast for instance uint pointers to
union pointers as long as union contains a uint type, hence use this with some
callers, other just use union util_color in the first place.