drm/amdgpu/discovery: reserve discovery data at the top of VRAM

IP Discovery data is TMR fenced by the latest PSP BL,
so we need to reserve this region.

Tested on navi10/12/14 with VBIOS integrated with latest PSP BL.

v2: use DISCOVERY_TMR_SIZE macro as bo size
    use amdgpu_bo_create_kernel_at() to allocate bo

Signed-off-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>