drm/i915: Print PCI revision in i915_dump_device_info()

Knowing the device stepping may be crucial in analyzing problems. Since
we always ask bug reporters for dmegs with drm.debug=0xe (or something)
it would be nice if the PCI revision is already included in the dump.
Avoids having to ask for lspci output as well.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>