drm/i915/dsi: Add cmd mode flags in display mode private flags

Adding TE flags and periodic command mode flags
as part of private flags to indicate what TE interrupts
we would be getting instead of vblanks in case of mipi dsi
command mode.

v2: Add TE flag description (Jani)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200312053841.2794-4-vandita.kulkarni@intel.com