drm: drm_irq.h header cleanup

- Drop extern for functions, it's noise.
- Move&consolidate drm.ko internal parts into drm-internal.h.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161114090255.31595-6-daniel.vetter@ffwll.ch