drm/amdgpu: move si_dma.c away from sid.h and si_enums.h

Replace defines for the ones in oss_1_0_d.h and oss_1_0_sh_mask.h

Taking the opportunity to add some comments taken from cik_sdma.c

Signed-off-by: Alexandre Demers <alexandre.f.demers@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>