gallium: Make sure we return is_unorm/is_snorm for compressed formats.

The util helpers were looking for a non-void channels in a non-mixed format and returning its snorm/unorm state. However, compressed formats don't have non-void channels, so they always returned false. V3D wants to use util_format_is_[su]norm for its border color clamping workarounds, so fix the functions to return the right answer for these.

This now means that we ignore .is_mixed. I could retain the is_mixed check, but it doesn't seem like a useful feature -- the only code I could find that might care is freedreno's blit, which has some notes about how things are wonky in this area anyway.