vp9_dx_iface: move struct defs to separate header

this avoids redefining vpx_codec_vp9_dx, vpx_codec_vp9_dx_algo in
vp9_encoder_parms_get_to_decoder.cc

Change-Id: I3b89e7a62497227ee32419f1a7d30e4c10a13c05
(cherry picked from commit ca163b85)