gma500: Disable the clock gating of display controller to make DP/eDP work well

I don't know why the DP/eDP is affected by the clock gating. But the test
shows that it really fixes the DP/eDP clock issue during enabling DP/eDP.

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
[Updated to only apply the workaround if the device has DP. We don't want
 to do this on netbooks]
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>