anv: Advertise variableMultisampleRate

Initially, I didn't understand this feature.  Turns out that all it
means is that you can switch multisample rates in the middle of a
zero-attachment subpass.  We've been able to do this since forever.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>

src/intel/vulkan/anv_device.c

@@ -761,7 +761,7 @@ void anv_GetPhysicalDeviceFeatures(
                                                   pdevice->info.has_64bit_types,
       .shaderInt16                              = false,
       .shaderResourceMinLod                     = false,
-      .variableMultisampleRate                  = false,
+      .variableMultisampleRate                  = true,
       .inheritedQueries                         = true,
    };