drm/i915/chv: fix HW readout of the port PLL fractional divider

Ville noticed that the PLL HW readout code parsed the fractional
divider value as if the fractional divider was always enabled. This may
result in a port clock state check mismatch if the preceeding modeset
disabled the fractional divider, but left a non-zero divider value in
the register.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>