drm/i915/dsi: Define command mode registers

Adding all the register definitions needed
for mipi dsi command mode.

Signed-off-by: Madhav Chauhan <madhav.chauhan@intel.com>
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/20191111111029.9126-2-vandita.kulkarni@intel.com