drm/vc4: Correct DSI register definition

The DSI1_PHY_AFEC0_PD_DLANE1 and DSI1_PHY_AFEC0_PD_DLANE3 register
definitions were swapped, so trying to use more than a single data
lane failed as lane 1 would get powered down.
(In theory a 4 lane device would work as all lanes would remain
powered).

Correct the definitions.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>