drm: remove the DRM pci domain

This patch removes the pci_domain from the DRM device structure, and
gets it via a macro that either asks the platform or does the alpha special
case. jgarzik asked for this to just use the platform magic, but I've no
alpha experience and I'd rather not just break it and wait for someone to
give out.

Signed-off-by: Dave Airlie <airlied@linux.ie>