drm/i915/tgl: Fix the read of the DDI that transcoder is attached to

drm/i915/tgl: Fix the read of the DDI that transcoder is attached to

On TGL this register do not map directly to port, it was already
handled when setting it(TGL_TRANS_DDI_SELECT_PORT()) but not when
reading it.

To make it consisntent adding a macro for the older gens too.

v2:
Adding TGL_PORT_TRANS_DDI_SELECT() so all future users can reuse it
(Lucas)

v3:
Missed parentheses arround val (Jose)

v4:
Renamed TGL_PORT_TRANS_DDI_SELECT to TGL_TRANS_DDI_FUNC_CTL_VAL_TO_PORT
(Lucas)
Added TRANS_DDI_FUNC_CTL_VAL_TO_PORT (Lucas)

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190808004935.1787-2-jose.souza@intel.com