drm/amdgpu: cleanup adjust_mc_addr handling v4

Rename adjust_mc_addr to get_vm_pde and check the address bits in one place.

v2: handle vcn as well, keep setting the valid bit manually,
    add a BUG_ON() for GMC v6, v7 and v8 as well.
v3: handle vcn_v1_0_enc_ring_emit_vm_flush as well.
v4: fix the BUG_ON mask for GFX6-8

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>