drm/i915/dsi: Configure transcoder operation for command mode.

Configure the transcoder to operate in TE GATE command mode
and  take TE events from GPIO.
Also disable the periodic command mode, that GOP would have
programmed.

v2: Disable util pin (Jani)
v3: Use intel_de_write (Jani)

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/20200312053841.2794-2-vandita.kulkarni@intel.com