drm/i915: add intel_dp_to_dev and intel_hdmi_to_dev

When we add struct intel_digital_port, there will be no direct way of
going from intel_{dp,hdmi} to drm_device: we will need to call
container_of().

This patch adds functions to go from intel_{dp,hdmi} to drm_device.
The main goal here is to greatly reduce the size of the next patch,
where we will change the implementation of the functions we just
added here (among other things).

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