drm: inline drm_pci_set_unique

This is only used for drm versions 1.0, and kms drivers have never
been there. So we can appropriately restrict this to legacy and hence
pci devices and inline everything.

v2: Make the dummy function actually return something, caught by Wu
Fengguang's 0-day tester.

v3: Fix spelling in comment (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>