drm/i915/dsi: Move disable pll call outside of clear_device_ready()

On enable intel_dsi_enable() directly calls intel_enable_dsi_pll(),
make intel_dsi_disable() also directly call intel_disable_dsi_pll(),
rather then hiding the call in intel_dsi_clear_device_ready(),
no functional changes.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161201202925.12220-7-hdegoede@redhat.com