egl: new attempt to extend EGL_EXT_device_drm_render_node for split display/render SoCs

This is another attempt to extend EGL_EXT_device_drm_render_node for split display/render SoCs. !12796 (closed) is blocked on issues that we probably can't handle, so I've tried to fix that using the approach described in #5591 (closed).

For my surprise, I've found out that things are working by accident on the case of EGL GBM platform + KMS-only device. Currently, there are only EGLDevice's for render-capable devices on the global EGLDevice list. So for EGL GBM platform + KMS-only device, _eglFindDevice() should return NULL and things should fail, but _eglFindDevice() is returning the last element from the list when it can't find an EGLDevice.

cc: @daniels @emersion @eric @xexaxo

Depends on !25504 (merged)

Closes #5591 (closed)