drm/amd/amdgpu: sdma_v4_0_start: initialize r

Reported by smatch:
drivers/gpu/drm/amd/amdgpu/sdma_v4_0.c:1167 sdma_v4_0_start() error: uninitialized symbol 'r'.

Signed-off-by: Ernst Sjöstrand <ernstp@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>