drm/amdgpu: Do not program AGP BAR regs under SRIOV in gfxhub_v1_0.c

[ Upstream commit 057fef20 ]

SRIOV VF does not have write access to AGP BAR regs.
Skip the writes to avoid a dmesg warning.

Signed-off-by: Victor Lu <victorchengchi.lu@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>