drm/i915/dsi: Call MIPI_SEQ_TEAR_ON and DISPLAY_ON for cmd-mode (untested)

According to the spec we should call MIPI_SEQ_TEAR_ON and DISPLAY_ON
on enable for cmd-mode, just like we already call their counterparts
on disable. Note: untested, my panel is a vid-mode panel.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Bob Paauwe <bob.j.paauwe@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1488374106-4949-10-git-send-email-jani.nikula@intel.com