drm/i915/dp: Print full branch/sink descriptor

Extend the branch/sink descriptor info with the missing device ID
field. While at it also read out all the descriptor registers in one
transfer and make the debug print more compact.

v2: (Jani)
- Cache the descriptor in intel_dp.
- Split out this change into a separate patch.
v3: (Jani)
- Fix return value check of __intel_dp_read_desc().
- Use %pE instead of %s to print the device ID.

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1477401159-15098-1-git-send-email-imre.deak@intel.com