drm/i915: get a runtime PM reference when the panel VDD is on

And put it when it's off. Otherwise, when you run pm_pc8 from
intel-gpu-tools, and the delayed function that disables VDD runs,
we'll get some messages saying we're touching registers while the HW
is suspended.

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