drm/i915: Move lowfreq_avail around a bit in ilk/hsw_crtc_mode_set

Now this really should be in the pipe config somewhere, but till now
it isn't. We can at least move it up a bit next to all the other pll
code since intel_dp_set_m_n really doesn't depend upon this.

This is just prep work so that moving all the hw frobbing code from
->crtc_mode_set to ->crtc_enable is clean.

v2: Do the same for haswell while at it, not just for ivb.

Reviewed-by: Shobhit Kumar <shobhit.kumar@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>