drm/amdgpu: Fix DPX valid mode check on GC 9.4.3

For DPX mode, the number of memory partitions supported should be less
than or equal to 2.

Fixes: 1589c82a ("drm/amdgpu: Check memory ranges for valid xcp mode")
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>
(cherry picked from commit 990c4f58)
Cc: stable@vger.kernel.org