GL hangs after acc098a7 on newer amd GCN cards

After acc098a7 when I run the check-gst-pugins-base.pipeleines_gl* tests it hangs the machine. ssh and Pulseaudio are still responsive and it looks like its the graphics driver that has hang up.

Reverting the commit makes the hangs go away. This specific machine is a raven ridge apu with the amdgpu driver so it possible that its just a dirver bug since the hw and driver are quite new. The configuration tested is Fedora 29 with kenrel 4.19.14 and mesa 18.2.8.

04:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Raven Ridge [Radeon Vega Series / Radeon Vega Mobile Series] (rev c3) (prog-if 00 [VGA controller])
	Subsystem: Lenovo Device 3801
	Flags: bus master, fast devsel, latency 0, IRQ 71
	Memory at c0000000 (64-bit, prefetchable) [size=256M]
	Memory at d0000000 (64-bit, prefetchable) [size=2M]
	I/O ports at 1000 [size=256]
	Memory at d0600000 (32-bit, non-prefetchable) [size=512K]
	Capabilities: <access denied>
	Kernel driver in use: amdgpu
	Kernel modules: amdgpu

I asked @slomo to try and reproduce the hang, but was not able to do so with an igpu with the i965 driver. I will try to troubleshoot further in the coming days.

To reproduce:

# clone gst-build or from an existing checkout
meson build && ninja -C build
./gst-uninstalled.py gst-validate-launcher check.gst-plugins-base.pipelines_gl* -fs --mute --dump-on-failure --no-display --meson-no-rebuild