zink: force explicit fence only on first frame flush

we have implicit sync hooked up for drivers now so we don't need to worry
about drawing over our frontbuffer unexpectedly

still a weird issue remaining where we miss the first frame without an explicit
fence, but I expect that will get resolved eventually by wsi
39 jobs for !9290 with zink-defencing in 9 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Sanity
passed #7617893
sanity

00:00:09

 
  Container
manual #7617896
aarch64 manual
arm_build
manual #7617897
manual
arm_test-base
manual #7617898
windows shell 1809 mesa manual
windows_build_vs2019
manual #7617894
manual
x86_build-base
manual #7617895
manual
x86_test-base
 
  Container 2
created #7617906
android_build
created #7617909
arm64_test
created #7617910
armhf_test
created #7617903
i386_build
created #7617899
kernel+rootfs_amd64
created #7617900
aarch64
kernel+rootfs_arm64
created #7617901
aarch64
kernel+rootfs_armhf
created #7617904
ppc64el_build
created #7617905
s390x_build
created #7617902
x86_build
created #7617907
x86_test-gl
created #7617908
x86_test-vk
 
  Meson X86 64
created #7617916
meson-clang
created #7617917
meson-clover
created #7617913
meson-clover-testing
created #7617914
gstreamer
meson-gallium
created #7617915
meson-release
created #7617911
meson-testing
created #7617912
meson-testing-asan
created #7617918
meson-vulkan
 
  Build Misc
created #7617919
meson-android
created #7617921
aarch64
meson-arm64
created #7617922
aarch64
meson-arm64-asan
created #7617923
aarch64
meson-arm64-build-test
created #7617920
aarch64
meson-armhf
created #7617924
meson-i386
created #7617927
meson-mingw32-x86_64
created #7617926
kvm
meson-ppc64el
created #7617925
kvm
meson-s390x
 
  Layered Backends
created #7617931
zink-lvp-gl
created #7617929
zink-lvp-gles2
created #7617930
zink-lvp-gles3
created #7617928
zink-piglit-quick_gl