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>