disp: msm: dsi: move backlight operations to post kickoff

This change moves the backlight update operation from
drm bridge enable to connector post kickoff.

When timing engine is enabled with programmable fetch
enabled, the timing engine will start counting from
the prog_fetch_start point (which is somewhere in VFP).
It’s a grey area from that point to the actual panel
vsync and SW should not trigger DMA command during that
time.

During display resume, sometimes the INTF timing engine
do not get enabled completely at the first vblank irq.
The backlight update cmd transfer trigger as part of the
drm bridge enable can also take place at the same time,
that may cause DSI cmd transfer failure.

Change-Id: I2722d3c23012ef0e7bcc7f728ec5658318ce4e60
Signed-off-by: Rajeev Nandan <rajeevny@codeaurora.org>