com.canonical.certification::suspend/{index}_gl_support_after_suspend_{product_slug} fails if the desktop contains two GPUs and the output is from add-on graphic card

* Steps to reproduce:
checkbox-cli run com.canonical.certification::device com.canonical.certification::graphics_card com.canonical.certification::suspend/2_gl_support_after_suspend_Oland__Radeon_HD_8570___R7_240_340___Radeon_520_OEM__auto

* Description:
It could get the positive result when
1. systemctl suspend
2. /usr/lib/nux/unity_support_test -p 2>&1
```
OpenGL vendor string: X.Org
OpenGL renderer string: AMD OLAND (DRM 2.50.0, 5.6.0-1036-oem, LLVM 10.0.0)
OpenGL version string: 4.5 (Compatibility Profile) Mesa 20.0.8

Not software rendered: yes
Not blacklisted: yes
GLX fbconfig: yes
GLX texture from pixmap: yes
GL npot or rect textures: yes
GL vertex program: yes
GL fragment program: yes
GL vertex buffer object: yes
GL framebuffer object: yes
GL version is 1.4+: yes

Unity 3D supported: yes
```

But if I ran the command from "2_gl_support_after_suspend_Oland__Radeon_HD_8570___R7_240_340___Radeon_520_OEM__auto" the result will be
```
--------------[ Running job 6 / 6. Estimated time left: unknown ]---------------
[ Test OpenGL support for Oland [Radeon HD 8570 / R7 240/340 / Radeon 520 OEM] after suspend ]
ID: com.canonical.certification::suspend/2_gl_support_after_suspend_Oland__Radeon_HD_8570___R7_240_340___Radeon_520_OEM__auto
Category: com.canonical.plainbox::suspend
... 8< -------------------------------------------------------------------------
Setting up PRIME GPU offloading for AMD discrete GPU
OpenGL vendor string: Intel
OpenGL renderer string: Mesa Intel(R) UHD Graphics 630 (CML GT2)
OpenGL version string: 4.6 (Compatibility Profile) Mesa 20.0.8

Not software rendered: yes
Not blacklisted: yes
GLX fbconfig: yes
GLX texture from pixmap: no
GL npot or rect textures: yes
GL vertex program: yes
GL fragment program: yes
GL vertex buffer object: yes
GL framebuffer object: yes
GL version is 1.4+: yes

Unity 3D supported: no
------------------------------------------------------------------------- >8 ---
Outcome: job failed
```

BTW, it's a cert blocker issue. Set the importance to Critical.