drm/i915/dsi: remove intel_dsi_cmd.c and the unused functions therein

The removed functions can be resurrected in intel_dsi.c as need arises.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-By: Shobhit Kumar <shobhit.kumar@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>