drm: populate irq_by_busid-member for pci

Commit 8410ea (drm: rework PCI/platform driver interface) implemented
drm_pci_irq_by_busid() but forgot to make it available in the
drm_pci_bus-struct.

This caused a freeze on my Radeon9600-equipped laptop when executing glxgears.
Thanks to Michel for noticing the flaw.

[airlied: made function static also]

Reported-by: Michel Dänzer <daenzer@vmware.com>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>