drm/i915: Read DP branch device SW revision

drm/i915: Read DP branch device SW revision

SW revision is mandatory field for DisplayPort branch
devices. This is defined in DPCD register fields 0x50A
and 0x50B.

v2: move drm_dp_ds_revision structure to be part of
    drm_dp_link structure (Daniel)
v3: remove dependency to drm_dp_helper but instead parse
    DPCD and print SW revision info to dmesg (Ville)
v4: commit message fix (Jim Bride)

Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Jim Bride <jim.bride@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1473419458-17080-9-git-send-email-mika.kahola@intel.com