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

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.

Reviewed-by: <Roland Scheidegger sroland@vmware.com>