nv30: U8_USCALED only works for size 4

See https://bugs.freedesktop.org/show_bug.cgi?id=61635 for a sample
program. Changing it to use a vec4 makes it work. Remove the unsupported
formats.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "9.2 and 9.1" <mesa-stable@lists.freedesktop.org>