drm/amdgpu: add generic display panic helper code

Pull this out of Jocelyn's patch and make it generic.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Lu Yao <yaolu@kylinos.cn>
Cc: Jocelyn Falempe <jfalempe@redhat.com>
Cc: Harry Wentland <harry.wentland@amd.com>