gallium/util: use ui[4] instead of ui in union util_color

gallium/util: use ui[4] instead of ui in union util_color

util_color often merely represents a collection of bytes, however it is
inconvenient if those bytes can only be accessed as floats/doubles for int
formats exceeding 32bits.
(Note that since rgba8 formats use one uint, not 4 bytes, hence the byte and
short member were left as is.)