ui: gbm_wrapper: lower severity of log message

On ChromeOS, not being able to create a GBM BO for a buffer prevents us
from using hardware overlays with this buffer, but is not a severe error
in itself. However, currently a warning is unconditionally displayed in
both debug and release builds, which tends to accumulate with time and
suggests that something is seriously going wrong:

gbm_wrapper.cc(286)] Failed to create GBM BO, YV12, 320x240, flags: 0x2021; gbm_device_is_format_supported() = 0

Turn this LOG(WARNING) into a DVLOG(2), so only developers explicitly
interested in understanding what exactly happens at the GBM level are
notified of this failure.

BUG=b:143323148
TEST=VDA unittest builds and runs on Hana/Kukui in Debug mode. Confirmed
     that the log did not appear in regular runs.

Change-Id: Ide75e80455be790aa1bcadaa875945bbc42c35e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1974299
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726737}

ui/gfx/linux/gbm_wrapper.cc

@@ -283,10 +283,10 @@ class Device final : public ui::GbmDevice {
                                         format >> 16 & 0xFF,
                                         format >> 24 & 0xFF, 0};
 
-      LOG(WARNING) << "Failed to create GBM BO, " << fourcc_as_string << ", "
-                   << size.ToString() << ", flags: 0x" << std::hex << flags
-                   << "; gbm_device_is_format_supported() = "
-                   << gbm_device_is_format_supported(device_, format, flags);
+      DVLOG(2) << "Failed to create GBM BO, " << fourcc_as_string << ", "
+               << size.ToString() << ", flags: 0x" << std::hex << flags
+               << "; gbm_device_is_format_supported() = "
+               << gbm_device_is_format_supported(device_, format, flags);
 #endif
       return nullptr;
     }