drm/i915: Don't multiply the watermark latency values too early

The LP1+ watermark latency values need to be multiplied by 5 to
make them suitable for watermark calculations. However on pre-HSW
platforms we're going to need the raw value later when we have to
write it to the WM_LPn registers' latency field. So delay the
multiplication until it's needed.

Note: Paulo complains that the units of wm (now in 100ns) aren't
really clear and I agree. But that can be fixed later on ...

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: Add a comment about the unit obfuscation.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>