Encoders are not alike, make enable and disable hooks optional like
other hooks. Utilize this in DSI code, and remove the silly nop hook.

v2: Add the check also to intel_sanitize_encoder() (Madhav)
Reviewed-by: NMadhav Chauhan <madhav.chauhan@intel.com>
Acked-by: NVille Syrjala <ville.syrjala@linux.intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181016124134.10257-1-jani.nikula@intel.com