egl: add EGL_EXT_device_drm support

egl: add EGL_EXT_device_drm support

Add implementation based around the drmDevice API. As such it's only
available only when building with libdrm. With the latter already a
requirement when using !SW code paths in the platform code.

Note: the current code will work if a device is hot-plugged. Yet
hot-unplugged is not implemented, since I have no ways of testing it.

v2:
 - ddd some _eglDeviceSupports checks
 - require DRM_NODE_RENDER
 - add _eglGetDRMDeviceRenderNode helper

v3:
 - flip inverted asserts (Mathias)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Mathias Fröhlich <Mathias.Froehlich@web.de>