drm/i915: Don't pretend SDVO hotplug works on 915

915 doens't support hotplug at all, so we shouldn't try to pretend
otherwise in the SDVO code.

Note: i915 does have hotplug support in hw, we simply never enabled it
in i915.ko: There's only one hpd bit for all outputs, so not worth the
bother to add this special case for this rather old platform.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
[danvet: Clarify that only i915.ko doesn't support hpd on i915g.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>