drm/amdkfd: Fix NULL pointer dereference in svm_migrate_to_ram()

./drivers/gpu/drm/amd/amdkfd/kfd_migrate.c:985:58-62: ERROR: p is NULL but dereferenced.

Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=2549
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org