drm/i915: Avoid div-by-zero in clock calculation funcs

Check that the N and P dividers don't cause a divide by zero.

This shouldn't happen under normal circumstances, but can
happen eg. under simulation.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>