drm/i915: Update drm/i915 bug filing URL

We've moved from bugzilla to gitlab.

Cc: stable@vger.kernel.org
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200212160434.6437-2-jani.nikula@intel.com