devices: gpu: fallback to 2d rendering if virgl is not enabled

The virgl renderer was unconditionally selected as the default backend
for crosvm, even if the "virgl_renderer" feature is not enabled. This
forces the user to explicitly specify the 2d backend with the --gpu
option, which can be confusing for new users.

Simplify things a bit by using the 2D renderer as default if the
"virgl_renderer" feature is not enabled - that way the GPU can be used
without having to specify a backend explicitly.

BUG=b:213532598
TEST=successfully run crosvm without the "virgl_renderer" feature and
without specifying the "backend" GPU option.

Change-Id: Ib36b7d92cef62d9dd91b0a41051362d1c57e0536
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3528233
Reviewed-by: Keiichi Watanabe <keiichiw@chromium.org>
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>

devices/src/virtio/gpu/mod.rs

@@ -119,7 +119,11 @@ impl Default for GpuParameters {
             gfxstream_use_guest_angle: false,
             gfxstream_use_syncfd: true,
             use_vulkan: false,
-            mode: GpuMode::ModeVirglRenderer,
+            mode: if cfg!(feature = "virgl_renderer") {
+                GpuMode::ModeVirglRenderer
+            } else {
+                GpuMode::Mode2D
+            },
             cache_path: None,
             cache_size: None,
             udmabuf: false,