drm/amdgpu:fix gmc_v9 vm fault process for SRIOV

for SRIOV we cannot use access register when in IRQ routine
with regular KIQ method

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>