drm: Give irq_by_busid drm_legacy_ prefix

It's the only ioctl handler purely for legacy drivers that didn't have
this yet.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201008142927.2819321-1-daniel.vetter@ffwll.ch