drm/i915: Remove intel_dsi_cmd.h

This header has been unusued since:

  commit 063c86f6
  Author: Jani Nikula <jani.nikula@intel.com>
  Date:   Fri Jan 16 14:27:27 2015 +0200

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

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>