drm/i915: Make pipe/transcoder offsets not depend on enum values

Depending on the transcoder enum values to translate from transcoder
to pipe/transcoder register addresses can easily break if we add a new
transcoder. So remove the dependency by using named initializers.

Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.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/20181120092325.21249-1-imre.deak@intel.com