drm/i915: factor out intel_edp_panel_vdd_sanitize

This will be needed by an upcoming patch too that needs to sanitize the
VDD state during resume. The additional async disabling is only needed
for the resume path, here it doesn't make a difference since we enable
VDD right after the sanitize call.

v2:
- don't set intel_dp ptr for non-eDP encoders (Ville)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>