drm/amdgpu/nbio4.3: set proper rmmio_remap.reg_offset for SR-IOV

[ Upstream commit ab43213e ]

Needed for HDP flush to work correctly.

Reviewed-by: Timmy Tsai <timmtsai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>