drm/i915/selftests: Pretend to be a gfx pci device

Set the class on our mock pci device to GFX. This should be useful for
utilities like intel-iommu that special case gfx devices.

References: https://bugs.freedesktop.org/show_bug.cgi?id=101080


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170518094638.5469-1-chris@chris-wilson.co.uk


Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>