drm/i915: simplify setting of ddi_io_power_domain

Instead of the ever growing switch, just compute the ddi io power domain
based on the port number.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191011010907.103309-2-lucas.demarchi@intel.com