drm/amdgpu: add initial support for sdma v5.0 (v6)

SDMA (System DMA) is a general purpose DMA engine usable
by UMDs for transfers or the kernel for paging or GPUVM
updates.

v1: support basic funcitonalites includes rb, ib, vm,
    copy buffer and trap irq
v2: convert to use new get_vm_pde in emit_vm_flush
v3: retire amdgpu_ttm_set_active_vram_size from sdma v5
v4: retire the redundant hdp_invalidate implementation
v5: squash in updates
v6: some golden regs moved to vbios

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>