drm/i915: Include the DRIVER_DATE in the error state

For forklifted frankenkernels, the reported kernel version has no
bearing on the actual driver version. Include our own driver date that
is updated every time we tag drm-intel-next.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190807113827.28127-1-chris@chris-wilson.co.uk