drm/i915: allow 945 to control self refresh (CxSR) automatically

I changed 945's self refresh to work without the need for the driver to
enable/disable self refresh manually based on the idle state of the gpu.
This is much better than enabling/disabling self refresh for various
reasons, including staying in a lower power state for more time and
avoiding the need for cpu cycles.

This was originally done manually to workaround issues with the hardware
hanging. However, since 94400120: drm/i915: enable low power render
writes on GEN3 hardware, automatic CxSR seems stable.

Signed-off-by: Alexander Lam <lambchop468@gmail.com>
Acked-by : Li Peng <peng.li@linux.intel.com>
[ickle: play safe with the ordering and disable CxSR before tweaking any
watermark and enable afterwards.]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>