etnaviv: fix incorrect varying interpolation

This corresponds to what the GC3000 blob does. The USED / UNUSED enums are
wrong, at least for GC2000/GC3000.

Without this the 3rd texture component is not interpolated correctly (flat?)
in the following test (and others):

dEQP-GLES2.functional.texture.mipmap.cube.generate.rgba8888_nicest

Strangely, when the texture is sampled from OpenGL it works correctly,
the problem only shows up for sampling by gallium/blitter. This fixes other
cube map tests which use util_blitter_blit.

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
13 jobs for !1137 with etna-texture in 11 minutes and 16 seconds (queued for 3 seconds)
detached
Status Job ID Name Coverage
  Containers Build
passed #419612
debian

00:02:44

 
  Build+Test
passed #419621
meson-arm64

00:02:23

passed #419620
meson-armhf

00:02:21

passed #419614
meson-clang

00:07:27

passed #419618
meson-clover

00:08:31

passed #419622
meson-i386

00:01:52

passed #419617
meson-main

00:05:22

passed #419613
meson-swr-glvnd

00:04:48

passed #419619
meson-vulkan

00:02:40

passed #419624
scons-llvm

00:03:48

passed #419623
scons-nollvm

00:04:38

passed #419615
scons-swr

00:05:28

passed #419616
scons-win64

00:06:10