drm/i915: cck reg used for checking DSI Pll locked

Instead of pipe configuration reg, cck reg to be used for checking whether
DSI Pll is getting locked or not.

v2: dpio_lock unlocked now in case DSI PLL lock fails

Signed-off-by: Gaurav K Singh <gaurav.k.singh@intel.com>
Signed-off-by: Shobhit Kumar <shobhit.kumar@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>