drm/vc4: dsi: Add correct stop condition to vc4_dsi_encoder_disable iteration

drm/vc4: dsi: 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.

Fixes: f2eb28c2170d ("drm/vc4: dsi: Fix bridge chain handling")
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://lore.kernel.org/r/20220613144800.326124-17-maxime@cerno.tech
Signed-off-by: Maxime Ripard <maxime@cerno.tech>