Ozone DRM: use GBM_BO_USE_HW_VIDEO_DECODER

This CL adds the GBM_BO_USE_HW_VIDEO_DECODER flag to
gfx::BufferUsage::SCANOUT_VDA_WRITE buffers. It's a spinoff of a
comment [1] in crrev.com/c/1021675.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1021675/18/ui/ozone/platform/drm/gpu/drm_thread.cc#147

Bug: 822346
Test: simplechrome video playback on nautilus, vaapi engaged.
Change-Id: I795c6c3f74574ffab60c24d1ef746b22d214120a
Reviewed-on: https://chromium-review.googlesource.com/1104394Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568027}

ui/ozone/platform/drm/gpu/drm_thread.cc

@@ -144,7 +144,8 @@ void DrmThread::CreateBuffer(gfx::AcceleratedWidget widget,
       flags = GBM_BO_USE_LINEAR | GBM_BO_USE_SCANOUT | GBM_BO_USE_TEXTURING;
       break;
     case gfx::BufferUsage::SCANOUT_VDA_WRITE:
-      flags = GBM_BO_USE_SCANOUT | GBM_BO_USE_TEXTURING;
+      flags = GBM_BO_USE_SCANOUT | GBM_BO_USE_TEXTURING |
+              GBM_BO_USE_HW_VIDEO_DECODER;
       break;
     case gfx::BufferUsage::GPU_READ_CPU_READ_WRITE:
     case gfx::BufferUsage::GPU_READ_CPU_READ_WRITE_PERSISTENT: