drm/amdgpu: Remove the unneeded result variable

Return the sdma_v6_0_start() directly instead of storing it in another
redundant variable.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: zhang songyi <zhang.songyi@zte.com.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>