drm/i915/dp: Read DP descriptor for eDP and LSPCON too

As for external DP sink and branch devices read and print the DP
descriptor for eDP and LSPCON devices as well to aid debugging.

v2:
- Split out this change to a separate patch. (Jani)

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/1477326811-30431-7-git-send-email-imre.deak@intel.com