drm/i915: Don't factor in pixel multplier when deriving dotclock from link clock and M/N values

We feed the non-multiplied clock to intel_link_compute_m_n(), so the
opposite operation should use the same order of operations. So we just
multiply by pixel_multiplier in the end now.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>