drm: kill drm_device->irq

Like the last patch but adds a macro to get at the irq value instead of
dereferencing pdev directly.  Should  make things easier for the BSD guys and
if we ever support non-PCI devices.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>