drm/dp: Factor out helper to distinguish between branch and sink devices

This check is open-coded in a few places, so it makes sense to simplify
things by having a helper for it similar to the rest of DPCD feature
helpers.

v2: (Jani)
- Move the helper to drm_dp_helper.h.
- Split out this change to a separate patch.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1477326811-30431-2-git-send-email-imre.deak@intel.com