gfxinfo: support separate GPU and display devices
On the embedded devices it is pretty typical to have separate display and GPU devices. Add support for representing this via the tags.
One of the options might be to enumerate /sys/class/drm/card*/device/of_node/compatible
and generate tag list using those files.
Note: It is necessary to enumerate all such files and still filter out duplicates with the GPUs detected by other means (e.g. via the GPU device nodes). For example on iMX platforms there are two different DRM devices (one for the Adreno/Imageon GPU, one for the IPU display unit), while on Qualcomm devices there is only one DRM device bound to the display device, GPU unit can be detected only via DT/gpu.