drm/i915: Make CHICKEN_TRANS reg not depend on enum value

Depending on the transcoder enum values to translate from transcoder to
the corresponding CHICKEN_TRANS register can easily break if we add a
new transcoder. Add an explicit mapping instead, by using helpers to
look up the register instance either by transcoder or port (since
unconveniently the registers have both port and transcoder specific
bits).

While at it also check for the correctness of GEN, port, transcoder. I
wasn't sure if psr2_enabled can only be set for GEN9+, but that seems to
be the case indeed (see setting of sink_psr2_support in
intel_psr_init_dpcd()).

v2 (Ville):
- Make gen9_chicken_trans_reg() internal to intel_psr.c.
- s/trans/cpu_transcoder/

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/20181119180021.370-1-imre.deak@intel.com