drm: always determine branch device with drm_dp_is_branch()

The helper should always be used.

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Oleg Vasilev <oleg.vasilev@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190829114854.1539-2-oleg.vasilev@intel.com