meson: allow "venus without virgl" builds

`cc: mesa-stable` instead of `fixes:` because several commits have
modified this but keeping this bug:
- 06e57e3231 ("virtio: Add vdrm native-context helper") made
  an unconditional copy of subdir(virtio)
- cede4e7ac3 ("meson: Only include virtio when DRM available")
  introduced a new condition, which doesn't cover everything that was
  needed
- other commits made more changes

Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35723>

src/meson.build

@@ -69,7 +69,7 @@ endif
 if with_any_intel
   subdir('intel')
 endif
-if system_has_kms_drm or with_gallium_virgl
+if system_has_kms_drm or with_gallium_virgl or with_virtio_vk
   subdir('virtio')
 endif
 if with_gallium_radeonsi or with_amd_vk or with_gallium_r300 or with_gallium_r600
@@ -90,9 +90,6 @@ endif
 if with_gallium_panfrost or with_gallium_lima or with_panfrost_vk or with_tools.contains('panfrost')
   subdir('panfrost')
 endif
-#if with_gallium_virgl or with_virtio_vk
-#  subdir('virtio')
-#endif
 if with_microsoft_clc or with_gallium_d3d12 or with_spirv_to_dxil or with_microsoft_vk
   subdir('microsoft')
 endif