drm/i915: BDW PSR: Remove DDIA limitation for Broadwell.

Broadwell has a PSR per transcoder, where DDIA supports
link disable and link standby modes while other
transcoders only support link standby.

Reviewed-by: Vijay Purushothaman <vijay.a.purushothaman@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>