drm/amdgpu/vcn: fix unitialized variable warnings

drm/amdgpu/vcn: fix unitialized variable warnings

Avoid returning an uninitialized value if we never enter the loop.
This case should never be hit in practice, but returning 0 doesn't
hurt.

The same fix is applied to the 4 places using the same pattern.

v2: - fixed typos in commit message (Alex)
    - use "return 0;" before the done label instead of initializing
      r to 0

Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>