drm/amdgpu: use static mmio offset for NV mailbox

what:
with the new "req_init_data" handshake we need to use mailbox
before do IP discovery, so in mxgpu_nv.c file the original
SOC15_REG method won'twork because that depends on IP discovery
complete first.

how:
so the solution is to always use static MMIO offset for NV+ mailbox
registers.
HW team confirm us all MAILBOX registers will be at the same
offset for all ASICs, no IP discovery needed for those registers

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Emily Deng <Emily.Deng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>