drm/i915/dp: Move port A pll setup to g4x_pre_enable_dp

drm/i915/dp: Move port A pll setup to g4x_pre_enable_dp

Only ilk/snb/ivb need the port A pll setup, so move it to the
pre_enable hook for those platforms. We can savely do this since on
those platforms there's nothing that touches the hardware between the
encoder->mode_set and the encoder->pre_enable calls.

Also add a comment that port A is ilk+ only.

Reviewed-by: Naresh Kumar Kachhi <naresh.kumar.kachhi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>