drm/vc4: Register dsi0 as the correct vc4 encoder type

vc4_dsi was registering both dsi0 and dsi1 as VC4_ENCODER_TYPE_DSI1
which seemed to work OK for a single DSI display, but fails
if there are two DSI displays connected.

Update to register the correct type.

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