drm/i915: Add code comment on assumption of pipe==transcoder

Add a comment to the pipe and transcoder enum definitions about our
assumption in the code about enum values for pipes and transcoders
with a 1:1 transcoder -> pipe mapping.

v2:
- Clarify more what are the assumptions about the enum values. (Ville)
v3: (Lucas)
- s/->/ -> / so it looks less like pointer dereferencing.
- Use pipe enums as initializers in the transcoder enum definition.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> (v2)
Link: https://patchwork.freedesktop.org/patch/msgid/20181120092325.21249-3-imre.deak@intel.com