drm/amdgpu: remove redundant logic related HDP

Remove hdp_flush function from amdgpu_nbio struct as it have been unified
into hdp struct.
Remove the include about hdp register which was not used.
V2: Remove hdp golden setting which is unnecessary.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>