drm/radeon/kms: properly power up/down the eDP panel as needed (v4)

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: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>