drm: Mark IRQ_BUSID ioctl as legacy

The functionality is only implemented for legacy drivers. Mark the ioctl
as legacy and move the code behind CONFIG_DRM_LEGACY. If legacy drivers
are disabled, the ioctl call now returns -EINVAL instead of -EOPNOTSUPP.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210516195135.3755-1-tzimmermann@suse.de