[platforms/drm] introduce env var to select GPU

Summary:
In some environemnts it is possible that udev detection of primary GPU
fails and it picks the first available card (normally card0). But it is
not desired, and it would be required to use different card (card1).

Introduce environment variable, which when used, will make it possible
to use different card node then one selected by udev.

You can use it like KWIN_DRM_DEVICE_NODE=/dev/dri/card1 kwin_wayland

Test Plan: Tested on the device where card0 was not usable for kms, but card1

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D24051