drm/amdkfd: Check correct memory types for is_system variable

To catch GPU mapping of system memory, TTM_PL_TT and AMDGPU_PL_PREEMPT
must be checked.

Fixes: 628e1ace ("drm/amdkfd: mark GFX12 system and peer GPU memory mappings as MTYPE_NC")
Signed-off-by: Sreekant Somasekharan <sreekant.somasekharan@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>