drm/i915/dsi: Add TE handler for dsi cmd mode.

In case of dual link, we get the TE on slave. So clear the TE on slave
DSI IIR.

If we are operating in TE_GATE mode, after we do a frame update, the
transcoder will send the frame data to the panel, after it receives a
TE. Whereas if we are operating in NO_GATE mode then the transcoder will
immediately send the frame data to the panel. We are not dealing with
the periodic command mode here.

v2: Pass only relevant masked bits to the handler (Jani)

v3: Fix the check for cmd mode in TE handler function.

v4: Use intel_handle_vblank instead of drm_handle_vblank (Jani)

v3: Use static on handler func (Jani)

Acked-by: Ville Syrjälä <ville.syrjala@linux.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/20200924124209.17916-4-vandita.kulkarni@intel.com