drm/amdgpu: Prevent kernel-infoleak in amdgpu_info_ioctl()

Compiler leaves a 4-byte hole near the end of `dev_info`, causing
amdgpu_info_ioctl() to copy uninitialized kernel stack memory to userspace
when `size` is greater than 356.

In 2015 we tried to fix this issue by doing `= {};` on `dev_info`, which
unfortunately does not initialize that 4-byte hole. Fix it by using
memset() instead.

Cc: stable@vger.kernel.org
Fixes: c193fa91 ("drm/amdgpu: information leak in amdgpu_info_ioctl()")
Fixes: d38ceaf9 ("drm/amdgpu: add core driver (v4)")
Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Peilin Ye <yepeilin.cs@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>