gpu: add force_ctx_0 calls prior to each gpu command

BUG=None
TEST=build with --features=gpu; null_platform_test

Change-Id: Ib7aab8b2f0d7e1523df49360ab99677b59ce6052
Reviewed-on: https://chromium-review.googlesource.com/1073960
Commit-Ready: David Riley <davidriley@chromium.org>
Tested-by: David Riley <davidriley@chromium.org>
Reviewed-by: Zach Reizner <zachr@chromium.org>

devices/src/virtio/gpu/backend.rs

@@ -937,4 +937,8 @@ impl Backend {
     pub fn fence_poll(&mut self) -> u32 {
         self.renderer.poll()
     }
+
+    pub fn force_ctx_0(&mut self) {
+        self.renderer.force_ctx_0();
+    }
 }

devices/src/virtio/gpu/mod.rs

@@ -92,6 +92,8 @@ impl Frontend {
                            cmd: GpuCommand,
                            data: Option<VolatileSlice>)
                            -> GpuResponse {
+        self.backend.force_ctx_0();
+
         match cmd {
             GpuCommand::GetDisplayInfo(_) => {
                 GpuResponse::OkDisplayInfo(self.backend.display_info().to_vec())

gpu_renderer/src/lib.rs

@@ -594,6 +594,10 @@ impl Renderer {
         };
         ret_to_res(ret)
     }
+
+    pub fn force_ctx_0(&self) {
+        unsafe { virgl_renderer_force_ctx_0() };
+    }
 }
 
 /// A context in which resources can be attached/detached and commands can be submitted.