drm/amdgpu: retire set_vga_state for some ASIC

set_vga_state operation only allowed on SI generation
ASIC, retire the realted function on those ASIC which
did not do anything.

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>