drm/amdgpu: add set_reg_remap callback for NBIO 7.7
This will be used to consolidate the register remap offset
configuration and fix HDP flushes on systems non-4K pages.
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>