drm/i915: update in-source bug filing URLs

The bug filing documentation has been moved from the gitlab wiki to
gitlab pages at https://drm.pages.freedesktop.org/intel-docs/.

Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231110114807.3455739-2-jani.nikula@intel.com