turnip: Advertise 8 bit subpixel precision

Previously, turnip advertised 4-bit subpixel precision when in
practice, a6xx seems to render with 8-bit precision. This caused
dEQP-VK.renderpass2.suballocation.subpass_dependencies.late_fragment_tests.*
to fail because they compare images rendered with turnip against
ones rendered via a software reference implementation parameterized
by turnip's VkPhysicalDeviceLimits.subPixelPrecisionBits value.

Tested-by: Marge Bot <mesa/mesa!4172>
Part-of: <mesa/mesa!4172>
28 jobs for !4172 with subpass-fixes in 18 minutes and 48 seconds (queued for 10 seconds)
detached
Status Name Job ID Coverage
  Container
passed arm_build #2175272
aarch64

00:01:02

passed arm_test #2175274
aarch64

00:00:36

passed windows_build_vs2019 #2175276
windows shell 1809 mesa

00:00:18

passed x86_build #2175265

00:02:27

passed x86_build_old #2175270

00:00:46

passed x86_test-gl #2175266

00:00:33

passed x86_test-vk #2175268

00:01:12

 
  Meson X86 64
passed meson-clang #2175285

00:08:06

passed meson-classic #2175283

00:03:25

passed meson-clover #2175287

00:06:07

passed meson-clover-old-llvm #2175288

00:05:41

passed meson-gallium #2175281

00:06:55

passed meson-testing #2175279

00:04:49

passed meson-vulkan #2175290

00:03:09

 
  Scons
passed scons-win64 #2175292

00:05:44

 
  Meson Misc
passed meson-arm64 #2175295
aarch64

00:05:26

passed meson-arm64-build-test #2175297
aarch64

00:02:35

passed meson-armhf #2175293
aarch64

00:05:45

passed meson-i386 #2175302

00:04:32

passed meson-mingw32-x86_64 #2175307

00:03:52

passed meson-ppc64el #2175305
kvm

00:05:24

passed meson-s390x #2175303
kvm

00:05:59

passed meson-windows-vs2019 #2175299
windows docker 1809 mesa

00:18:23

 
  Freedreno
passed arm64_a306_gles2 #2175316
google-freedreno-db410c

00:08:11

passed arm64_a530_gles2 #2175318
google-freedreno-db820c

00:09:41

passed arm64_a630_gles2 #2175309
mesa-cheza

00:01:33

passed arm64_a630_gles3 #2175314
mesa-cheza

00:05:37

passed arm64_a630_gles31 #2175311
mesa-cheza

00:04:14