drm/i915: only match on PCI_BASE_CLASS_DISPLAY

... not DISPLAY_VGA, because we ignore the VGA subclass with our
class_mask.

It confused me until Chris Wilson clued me up.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Keith Packard <keithp@keithp.com>