drm/i915: Do not wait atomically for display clocks

Looks like this code does not need to wait atomically since it
otherwise takes the mutex.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1457015805-23742-1-git-send-email-tvrtko.ursulin@linux.intel.com