drm/i915: tell the user KMS is required for gen6+

Educate the users why i915 won't load on gen6+ and nomodeset.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=61671
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>