drm/amdgpu: cleanup error handling in amdgpu_cs_parser_bos

Return early on success and so remove all those "if (r)" in the error
path.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>