drm/i915: set the IPS linetime watermark

Remove the "placeholder" comment and set the actual value described by
the specification. We still don't enable IPS, but it won't hurt to
already have the value set here.

While at it, fully set the register value instead of just masking the
values we're changing.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Resolve conflict due to reordered patches.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>