drm/radeon: Fix scratch register leak in IB test.

Restructure the code to jump out via labels instead of directly returning
early. Also make error reporting consistent across all hardware generations.

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Simon Kitching <skitching@vonos.net>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>