drm/irq: drm_legacy_ prefix for legacy ioctls

Spotted while auditing our ioctl table. Also nuke the
not-really-kerneldoc comments, we don't document internals and
definitely don't want to mislead people with the old dragons.

I think with this all the legacy ioctls now have proper drm_legacy_
prefixes.

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161213230814.19598-2-daniel.vetter@ffwll.ch