drm/amdgpu: correct ROM_INDEX/DATA offset for VEGA20

The ROMC_INDEX/DATA offset was changed to e4/e5 since
from smuio_v11 (vega20/arcturus).

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Tested-by: Candice Li <Candice.Li@amd.com>
Reviewed-by: Candice Li <Candice.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>