freedreno: Don't do texture prefetch for samples after a discard.

My theory is that it's more important that we avoid the memory traffic on
pixels that would be discarded than that we pipeline the fetches better.

I'm seeing lots of shaders affected in shader-db (ETQW, GLBenchmark 2.7
(t-rex), tesseract), so hopefully this will show up easily in benchmarks,
but it's not doing anything significant in my renderdoc-traces collection
so far.
28 jobs for !4309 with fd-prefetch-discard in 12 minutes and 25 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Container
passed #2046108
aarch64
arm_build

00:00:22

passed #2046109
aarch64
arm_test

00:00:20

passed #2046104
x86_build

00:00:17

passed #2046107
x86_build_old

00:00:18

passed #2046105
x86_test-gl

00:00:21

passed #2046106
x86_test-vk

00:00:17

 
  Meson X86 64
passed #2046113
meson-clang

00:08:18

passed #2046112
meson-classic

00:03:58

passed #2046114
meson-clover

00:03:39

passed #2046115
meson-clover-old-llvm

00:06:10

passed #2046111
meson-gallium

00:06:53

passed #2046110
meson-testing

00:06:34

passed #2046116
meson-vulkan

00:02:44

 
  Scons
passed #2046118
scons

00:12:04

passed #2046119
scons-old-llvm

00:04:57

passed #2046117
scons-win64

00:08:35

 
  Meson Misc
passed #2046121
aarch64
meson-arm64

00:01:35

passed #2046122
aarch64
meson-arm64-build-test

00:04:58

passed #2046120
aarch64
meson-armhf

00:02:09

passed #2046123
meson-i386

00:01:20

passed #2046126
meson-mingw32-x86_64

00:00:37

passed #2046125
kvm
meson-ppc64el

00:04:52

passed #2046124
kvm
meson-s390x

00:05:15

 
  Freedreno
passed #2046130
google-freedreno-db410c
arm64_a306_gles2

00:06:42

passed #2046131
google-freedreno-db820c
arm64_a530_gles2

00:07:15

passed #2046127
mesa-cheza
arm64_a630_gles2

00:01:47

passed #2046129
mesa-cheza
arm64_a630_gles3

00:06:14

passed #2046128
mesa-cheza
arm64_a630_gles31

00:04:10