The eDP panel must be powered up for aux transactions, so power it
up for detect and mode probe functions, otherwise power it up or
down based on dpms.

v2:
- only mess with eDP panel on DCE4+
- only mess with eDP panel on eDP connectors, not all DP connectors
v3:
- be extra careful to only mess with eDP panels on eDP connectors
v4:
- avoid possible null derefernce if a connector has not been
assigned to the encoder
Signed-off-by: NAlex Deucher <alexdeucher@gmail.com>
Signed-off-by: NDave Airlie <airlied@redhat.com>