drm/i915: ddi: move pch cleanup before encoder->post_disable

This is needed by an upcoming patch that moves the PCH/CRT PLL disabling
into the post_disable hook, after which we want to keep the modeset
sequence at its current state. At this point this won't have an effect
since the PCH/CRT post_disable hook is atm a NOP.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>