drm/i915: make encoder enable and disable hooks optional

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: Madhav Chauhan <madhav.chauhan@intel.com>
Acked-by: Ville Syrjala <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181016124134.10257-1-jani.nikula@intel.com