drm/amdgpu: Eliminate the set_pde_pte function pointer in amdgpu_gmc_funcs

All the gmc_*_set_pde_pte functions are the same across different ASICs,
so we can eliminate the set_pde_pte function pointer and instead use a
generic function.

Signed-off-by: Yong Zhao <Yong.Zhao@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>