eglfs_kms (GBM) should use eglGetPlatformDisplay when available

The GBM version of eglfs' DRM/KMS backend has some problems on some modern systems, see f.ex. https://github.com/lirios/wayland/commit/06bc9cb732895c101a1d9615959eb8eee3523a06.

For details see https://github.com/libretro/RetroArch/issues/4790

While the problem is arguably not Qt's fault, eglfs_kms could start using eglGetPlatformDisplay with EGL_PLATFORM_GBM_KHR, when available. This needs dynamic resolving of the EGL 1.5 function.

In fact the EGLDevice (NVIDIA) variant already does it in its customized QEglFSKmsEglDeviceIntegration::createDisplay. The GBM version should follow suit and do something similar.