radeon/uvd: fix field handling on R6XX style UVD

The first UVD generation can only do frame based output.

Signed-off-by: Christian König <christian.koenig@amd.com>
(cherry picked from commit 80771e47b6c1e47ab55f17311e1d4e227a9eb3d8)
Nominated-by: Alex Deucher <alexdeucher@gmail.com>