gma500: Program the DPLL lane based on the selected digitial port

Based on the spec, the CRT output doesn't use the lane. And the HDMI B output
uses the Lane0/1 while the HDMI C output uses the Lane 2/3. But currently
it will program all the four lanes for the CRT/HDMI.

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
[Ported to the in-kernel driver]
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>