Move DRM event queue related initialization to radeon_drm_queue_init

And make radeon_drm_queue_handler not directly accessible outside of
radeon_drm_queue.c.

(Ported from amdgpu commit 0148283984c77f7a6e97026edc3093497547e0a4)
Acked-by: Alex Deucher <alexander.deucher@amd.com>