drm/i915: Add comments about fixed pipe->transcoder/PLL mapping

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: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463059132-1720-3-git-send-email-imre.deak@intel.com