drm/i915: group link_standby setup and let this info visible everywhere.

No functional changes on this patch. Just grouping the link_standy decision
to avoid miss any change. Also making this info available everywhere
which will help to decide when to use vbt's tp time on following patch.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Durgadoss R <durgadoss.r@intel.com>
[danvet: Slight editing of the commit message which was one huge
run-on sentence.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>