radv: Fix SRGB compute copies.

SRGB stores are broken. We had compensation code in the
resolve path but none in the copy path. Since we don't
want any conversion and it does not matter for DCC,
just make everything UNORM instead.

This happened to cause wrong colors for the PRIME path, as
that uses image->buffer copies which always use the compute
path.

CC: 18.0 18.1 <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106587


Reviewed-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit a63a0960)