Drm GPU: small cleanups

Since QMap sorts its keys which does not make sense for pointer keys, use QHash. [1]

Use std::none_of somewhere it makes sense.

cc @zamundaaa @vladz

Both were suggested by clazy and cppcheck through Qtcreator.

[1] https://github.com/KDE/clazy/blob/master/docs/checks/README-qmap-with-pointer-key.md