drm/i915/sdvo: kfree the intel_sdvo_connector, not drm_connector, on destroy

Since the base fields in both struct intel_connector and struct
intel_sdvo_connector are at the beginning of the enclosing struct, the
pointers are essentially the same, but there is no requirement or guarantee
that this is always the case. Kfree the enclosing intel_sdvo_connector
pointer that was originally allocated, not the enclosed drm_connector, in
case someone ever rearranges the structs.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>