drm/i915/dp: split edp_panel_vdd_on() for reuse

Introduce _edp_panel_vdd_on() that returns true if the call enabled vdd,
and a matching disable is needed. Keep edp_panel_vdd_on() as a helper
for when it is expected the vdd is off.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>