gma500: Fix incorrect SR issue when disabling CRTC already in disabled state

Currently when trying to call the DPMS off again for one CRTC with DPMS off,
it will firstly disable the SR and can't enable it again because of the
incorrect check/logic. In such case the self refresh is still disabled
although one CRTC pipe is active. This is wrong.

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