drm/pci: fold in irq_by_busid support

This is a ums-only ioctl, and we've only ever supported ums (at least
in upstream) on pci devices. So no point in keeping that piece of
legacy logic abstracted within the drm bus driver.

To keep things work without CONFIG_PCI also add a dummy ioctl.

v2: Block the irq_by_busid ioctl for modeset drivers.

v3: Spelling/whitespace polish (Thierry)

Reviewed-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>