drm/i915: create intel_digital_port and use it

The goal is to have one single encoder capable of controlling both DP
and HDMI outputs. This patch just adds the initial infrastructure, no
functional changes.

Previously, both intel_dp and intel_hdmi were intel_encoders. Now,
these 2 structs do not have intel_encoder as members anymore. The new
struct intel_digital_port has intel_encoder as a member, and it also
includes intel_dp and intel_hdmi as members. In other words: see the
changes inside intel_drv.h: it's the most important change, everything
else is only to make it compile and work.

For now, each intel_digital_port is still only able to control one of
HDMI or DP, but not both together.

In the future we should also try to merge the common fields from
intel_dp and intel_hdmi (e.g., port).

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Add the missing ' ' spotted by Damien Lespiau.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>