drm/amdgpu: enable paging queue doorbell support v4

Because increase SDMA_DOORBELL_RANGE to add new SDMA doorbell for paging queue will
break SRIOV, instead we can reserve and map two doorbell pages for amdgpu, paging
queues doorbell index use same index as SDMA gfx queues index but on second page.

For Vega20, after we change doorbell layout to increase SDMA doorbell for 8 SDMA RLC
queues later, we could use new doorbell index for paging queue.

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