drm/amdgpu: NULL-check BO's backing store when determining GFX12 PTE flags

PRT BOs may not have any backing store, so bo->tbo.resource will be
NULL. Check for that before dereferencing.

Fixes: 0cce5f28 ("drm/amdkfd: Check correct memory types for is_system variable")
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Natalie Vock <natalie.vock@gmx.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>