drm/amdgpu: correctly toggle gfx on/off around RLC_SPM_* register access

As part of the ib padding process, accessing the RLC_SPM_* register may
trigger gfx hang. Since gfxoff may be already kicked during the whole period.
To address that, we manually toggle gfx on/off around the RLC_SPM_*
register access.

This can resolve the gfx hang issue observed on running Talos with RDP launched
in parallel.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Acked-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>