drm/i915: Calculate IPS linetime watermark based on future cdclk

Use the cdclk we're going to be using when the pipe gets enabled to
compute the IPS linetime watermark. The current cdclk frequency is
irrelevant at this point since it can still change.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461940278-17122-1-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>