drm/i915/dsi: Add check for periodic command mode

If the GOP has programmed periodic command mode,
we need to disable that which would need a
deconfigure and configure sequence.

v2: Fix sparse error, pass only intel_dsi (Jani)
v3: Use intel_de_read

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/20200312053841.2794-5-vandita.kulkarni@intel.com