VDPAU support appears to be broken on radeonsi as of git commit
0692ae34e939845e5185d3bdd33ddfe4afcdb995 "ac: move ac_get_num_physical_sgprs
into radeon_info". Trying to play back videos using MPV 0.29.1 with it enabled
fails with the following error within libvdpau_radeon: "LLVM failed to compile
a shader correctly: SGPR:VGPR usage is 56:20, but the hw limit is 0:0" (or
0:256 without the subsequent commit that does the same thing with
ac_get_num_physical_vgprs). I assume that information about the limits isn't
getting set properly for some reason.

Tested with a Radeon HD 7770 on Linux 5.2 with mpv 0.29.1, ffmpeg 4.2.1, the
radeon kernel driver, and xf86-video-ati 19.0.1.