Enable DP phy only when an alt mode got detected

This does not enable the phy until we get an event from the type-c chip (dusted off my extcon patches from march for that). This allows us to save power until the user uses an external screen. We can't currently power down again properly (#242) but this 'only' affects users using convergence. The upside is that this can get tested in tree without any additional device-tree magic.

i'll look at some numbers when runnning this on evg since my dogwood is flaky in this regard.

Closes: #233 (closed), #225 (closed)