drm/i915: Make EDP PSR flags not depend on enum values

Depending on the transcoder enum values to translate from transcoder
to EDP PSR flags can easily break if we add a new transcoder. So remove
the dependency by using an explicit mapping.

While at it also add a WARN for unexpected trancoders.

v2:
- Simplify things by defining flag shift values instead of indices.
- s/trans/cpu_transcoder/ (Ville)
v3:
- Define flags to look like separate bits instead of the values of
  the same bitfield. (Ville)

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>
Link: https://patchwork.freedesktop.org/patch/msgid/20181120092325.21249-2-imre.deak@intel.com