ozone: Remove USE_LINEAR when allocating modeset buffers.

crrev.com/2533163002 changed the way we allocate modeset buffers.
Unfortunately the new use flags introduced a regression that makes it
not possible to pageflip from a modeset buffer to a buffer queue buffer.
This CL restores the old modeset buffer use flags in order to fix the
regression, while we work on a long term solution.

BUG=673026
TEST=manually tried the step described in the bug and verified it now works.

Review-Url: https://codereview.chromium.org/2568243004
Cr-Commit-Position: refs/heads/master@{#438193}

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

@@ -40,8 +40,8 @@ class GbmBufferGenerator : public ScanoutBufferGenerator {
                                       uint32_t format,
                                       const gfx::Size& size) override {
     scoped_refptr<GbmDevice> gbm(static_cast<GbmDevice*>(drm.get()));
-    return GbmBuffer::CreateBuffer(gbm, format, size,
-                                   GBM_BO_USE_SCANOUT | GBM_BO_USE_LINEAR);
+    // TODO(dcastagna): Use GBM_BO_USE_MAP modifier once minigbm exposes it.
+    return GbmBuffer::CreateBuffer(gbm, format, size, GBM_BO_USE_SCANOUT);
   }
 
  protected: