Add a convenience macro for iterating DP encoders.
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180705164357.28512-2-ville.syrjala@linux.intel.comReviewed-by: NRodrigo Vivi <rodrigo.vivi@intel.com>