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

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 <[email protected]>
Cc: [email protected]
Signed-off-by: Imre Deak <[email protected]>
Reviewed-by: Jani Nikula <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]