mesa: drop iris/radeonsi/Vulkan drivers for greeter (need unported libclc)

Commit 08f3e71d41 enabled iris+radeonsi gallium drivers and Vulkan
(intel/amd/swrast); both pull -Dwith_clc -> dependency("libclc"), which is not
ported to Redox -> "meson.build:954: Dependency libclc not found". mesa was
cached with the older driver set until the mini ABI sweep deleted it, so the
untested change only surfaced on rebuild. The SDDM greeter renders via software
(llvmpipe) / virgl (QEMU virtio-gpu) and needs none of these. Revert to
softpipe,llvmpipe,virgl + no Vulkan so mesa builds. Re-enable hardware GPU
(iris/radeonsi/Vulkan) once libclc is ported.

local/recipes/libs/mesa/recipe.toml

@@ -91,14 +91,14 @@ cookbook_meson \
     -Degl=enabled \
     -Dgbm=enabled \
     -Dglx=disabled \
-    -Dgallium-drivers=softpipe,llvmpipe,virgl,iris,radeonsi \
+    -Dgallium-drivers=softpipe,llvmpipe,virgl \
     -Dllvm=enabled \
     -Dplatforms=wayland,redox \
     -Degl-native-platform=redox \
     -Dshader-cache=disabled \
     -Dc_args="['-include','alloca.h','-DCLOCK_MONOTONIC_RAW=(CLOCK_MONOTONIC+1000)','-Wno-error=implicit-function-declaration','-Wno-error=missing-prototypes','-Wno-error=return-type','-Wno-error=empty-body','-Wno-error=incompatible-pointer-types','-Wno-error=int-conversion','-Wno-error=format','-Wno-error','-std=gnu11','-Dstatic_assert=_Static_assert']" \
     -Dcpp_args="['-include','alloca.h','-Wno-error=implicit-function-declaration','-Wno-error=missing-prototypes','-Wno-error=return-type','-Wno-error=empty-body','-Wno-error=incompatible-pointer-types','-Wno-error=int-conversion','-Wno-error=format','-Wno-error']" \
-    -Dvulkan-drivers=intel,amd,swrast \
+    -Dvulkan-drivers= \
     -Dshared-glapi=enabled
 
 # Hack to add LLVM libs, the list can be seen from meson log and check for matches $("${LLVM_CONFIG}" --libs)