drm/i915: fix WRPLL clock calculation

Forgot to convert to using the refclk variable when I added refclk
readout support, and Paulo noticed the resulting calculation was off due
to the way p & r are stored.

Reported-by: Paulo Zanoni <przanoni@gmail.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>