Code checkers may complain about the explicit casts between different
enum types, so add comments for known-valid cases to help future
triaging of such complaints.

v2:
- Make the comments more logical (Ville).
Signed-off-by: NImre Deak <imre.deak@intel.com>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463059132-1720-3-git-send-email-imre.deak@intel.com