drm/amdgpu/sdma: Remove redundant lower_32_bits() calls when settings SDMA doorbell

Updated the patch for the pre-vega hardware. I kept the clamping code
to be safe.

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