drm/i915: clean up the cpu edp pll special case

By using the new pre_enable/post_disable functions.

To ensure that we only frob the cpu edp pll while the pipe is off add
the relevant asserts. Thanks to the new output state staging, this is
now really easy.

With this fixed we can now finally rip out the special-case handling
in the dp dpms code and replace it by the common intel_connector_dpms.

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