This adds hardware decoding for H.264 / HEVC / VP8 / VP9 using the MPP
Rockchip API.  It returns frames holding an AVDRMFrameDescriptor struct
in buf[0] that allows drm / dmabuf usage.  Tested on RK3288 (TinkerBoard)
and RK3328.
Signed-off-by: Mark Thompson <sw@jkqxz.net>