drm/irq: drm_control is a legacy ioctl, so pci devices only

This just adds a correspdonding check, follow-up patches will exploit
this.

v2: 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>

diff --cc drivers/gpu/drm/drm_irq.c
Simple merge