gallium/meson: Ensure all needed sym_config are set.

Move the sym_config setting all together and ensure that we set either
a value or empty, since Meson complains about missing values.

Fixes the build warning

```
   Configuring dri.sym using configuration
   ../src/gallium/targets/dri/meson.build:28: WARNING: The variable(s) 'va_driver_init',
     'vdp_imp_device_create_x11' in the input file 'src/gallium/targets/dri/dri.sym.in' are
     not present in the given configuration data.
```

Reviewed-by: Eric Engestrom <None>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32808>

src/gallium/meson.build

@@ -8,7 +8,8 @@ foreach d : [[with_gallium_r300 or with_gallium_radeonsi or with_gallium_r600, '
              [with_gallium_radeonsi, 'amdgpu_winsys_create'],
              [with_gallium_nouveau, 'nouveau_drm_screen_create'],
              [with_gallium_freedreno, 'fd_drm_screen_create_renderonly'],
-             [amd_with_llvm and with_gallium_radeonsi, 'ac_init_shared_llvm_once']]
+             [amd_with_llvm and with_gallium_radeonsi, 'ac_init_shared_llvm_once'],
+             [with_gallium_vdpau, 'vdp_imp_device_create_x11']]
   if d[0]
     sym_config.set(d[1], d[1] + ';')
   else
@@ -16,6 +17,12 @@ foreach d : [[with_gallium_r300 or with_gallium_radeonsi or with_gallium_r600, '
   endif
 endforeach
 
+if with_gallium_va
+  sym_config.set('va_driver_init', '__vaDriverInit_*_*;')
+else
+  sym_config.set('va_driver_init', '')
+endif
+
 inc_gallium_drivers = include_directories('drivers')
 inc_gallium_winsys = include_directories('winsys')
 inc_gallium_winsys_sw = include_directories('winsys/sw')

src/gallium/targets/dri/meson.build

@@ -17,11 +17,9 @@ if with_gallium_va or with_gallium_vdpau
   gallium_dri_link_with += [libgalliumvlwinsys]
   if with_gallium_va
     gallium_dri_link_whole += [libva_st]
-    sym_config.set('va_driver_init', '__vaDriverInit_*_*;')
   endif
   if with_gallium_vdpau
     gallium_dri_link_whole += [libvdpau_st]
-    sym_config.set('vdp_imp_device_create_x11', 'vdp_imp_device_create_x11;')
   endif
 endif