This allows creating the silo when starting the engine with custom
plugin keys such as WacomI2cFlashBaseAddr.
If we move the plugin initialization earlier then we don't get the
HwID matches, so we really do have to split this into a 4-stage startup,
e.g. ->load(), ->init(), ->startup() and ->coldplug().
We were calling g_module_symbol() 2703 times, which is actually more
expensive than you'd think.
It also means the plugins are actually what we tell people they are:
A set of vfuncs that get run. The reality before that they were dlsym'd
functions that get called at pretty random times.
There are now multiple plugins using drm_dp_aux_dev interface which
may potentially be combined with an amdgpu. Prevent exercising this
interface with any plugin using DP aux unless a new enough kernel is
installed.
