drm/i915/dsi: Use private flags to indicate TE in cmd mode

On dsi cmd mode we do not receive vblanks instead
we would get TE and these flags indicate TE is expected on
which port.

Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200312053841.2794-6-vandita.kulkarni@intel.com