drm/amdgpu: Use another offset for GC 9.4.3 remap

The legacy region at 0x7F000 maps to valid registers in GC 9.4.3 SOCs.
Use 0x1A000 offset instead as MMIO register remap region.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>