drm/vc4: Make DSI call into the bridge after the DSI link is enabled.

drm/vc4: Make DSI call into the bridge after the DSI link is enabled.

This allows panels or bridges that need to send DSI commands during
pre_enable() to successfully send them.  We delay DISP0 (aka the
actual display) enabling until after pre_enable so that pixels aren't
streaming before then.

v2: Just clear out the encoder->bridge value to disable the midlayer
    calls (idea by Andrzej Hajda).

Signed-off-by: Eric Anholt <eric@anholt.net>
Cc: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180621231759.29604-1-eric@anholt.net
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>