drm/vc4: Add correct stop condition to vc4_dsi_encoder_disable iteration

vc4_dsi_encoder_disable is partially an open coded version of
drm_bridge_chain_disable, but it missed a termination condition
in the loop for ->disable which meant that no post_disable
calls were made.

Add in the termination clause.

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