drm/i915/vlv: fix up broken precision in vlv_crtc_clock_get

With some divider values we end up with the wrong result.  So remove the
intermediates (like Ville suggested in the first place) to get the right
answer.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>