drm/amdgpu/sdma5: fix mask value of POLL_REGMEM packet for pipe sync

sdma will hang once sequence number to be polled reaches 0x1000_0000

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org