drm/amdgpu: Fix MMIO HDP flush on SRIOV

Disable HDP register remapping on SRIOV and set rmmio_remap.reg_offset
to the fixed address of the VF register for hdp_v*_flush_hdp.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Tested-by: Bokun Zhang <bokun.zhang@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>