drm/vc4: Move the DSI clock divider workaround closer to the clock call.

We want the adjusted_mode->clock to be the actual clock we're
expecting to program, so that consumers see the right values for clock
and vrefresh.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20170815234722.20700-1-eric@anholt.net


Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
(cherry picked from commit d409eeaf)