drm/amdgpu: Clean up gmc_v9_0_gart_enable

Many logic in this function are HDP set up,
not gart set up. Moved those logic to gmc_v9_0_hw_init.
No functional change.

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Acked-by: Christian konig <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>