drm/i915: Move Pineview CxSR and watermark code into update_wm hook.

Previously, after setting up the Pineview CxSR state, i9xx_update_wm would
get called and overwrite our state.

BTW: We will disable the self-refresh and never enable it any more if we
can't find the appropriate the latency on pineview plaftorm. In such case
the update_wm callback will be NULL.

The bitmask macro is also defined to access the corresponding fifo
watermark register.

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>