drm/i915/chv: Force PHY clock buffers off after PLL disable

drm/i915/chv: Force PHY clock buffers off after PLL disable

Now that we forced the clock buffers on in .pre_pll_enable() we
should probably undo the damage after we've turned the PLL off.

We do the clock buffer force enable in the .pre_pll_enable() hook
as we need to know which port is going to be used, but in the disable
case we don't need the port since we just disable the clock buffers
to both channels. So we can do this in chv_disable_pll() instead
of having to add any kind of .post_pll_disable() hook.

v2: Improve the commit message

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>