ozone/wayland: do not return supported buffer formats if gbm disabled.

if use_wayland_gbm set to false, Chromium doesn't use dmabuf path.
However, it still gets supported buffer formats that it got
from either wl_drm or zwp_linux_dmabuf protocols.

This is incorrect. Instead, it must report all the buffer formats
as "software only" (can be checked in the about://gpu page).

Bug: 578890
Change-Id: I44234af9b1b5b8960b2f698f815a62d95922660b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2404922
Commit-Queue: Robert Kroeger <rjkroege@chromium.org>
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Auto-Submit: Maksim Sisov (GMT+3) <msisov@igalia.com>
Cr-Commit-Position: refs/heads/master@{#806284}

ui/ozone/platform/wayland/host/wayland_buffer_manager_host.cc

@@ -662,12 +662,13 @@ void WaylandBufferManagerHost::OnChannelDestroyed() {
 
 wl::BufferFormatsWithModifiersMap
 WaylandBufferManagerHost::GetSupportedBufferFormats() const {
+#if defined(WAYLAND_GBM)
   if (connection_->zwp_dmabuf())
     return connection_->zwp_dmabuf()->supported_buffer_formats();
   else if (connection_->drm())
     return connection_->drm()->supported_buffer_formats();
-  else
-    return {};
+#endif
+  return {};
 }
 
 bool WaylandBufferManagerHost::SupportsDmabuf() const {