drm/bridge: ti-sn65dsi86: Fix power off sequence

When testing with a panel that's apparently a little more persnickety
about the correct power sequence (specifically Samsung ATNA33XC20), we
found that the ti-sn65dsi86 was doing things just slightly wrong.

Looking closely at the ti-sn65dsi86's datasheet, the power off
sequence is supposed to be:
1. Clear VSTREAM_ENABLE bit
2. Stop DSI stream from GPU. DSI lanes must be placed in LP11 state.
3. Program the ML_TX_MODE to 0x0 (OFF)
4. Program the DP_NUM_LANES register to 0x0
5. Clear the DP_PLL_EN bit.
6. Deassert the EN pin.
7. Remove power from supply pins

Since we were doing the whole sequence in the "disable", I believe
that step #2 (stopping the DSI stream from the GPU) wasn't
happening. We also weren't setting DP_NUM_LANES to 0.

Let's fix this.

NOTE: things are a little asymmetric now. For instance, we turn the
PLL on in "enable" but now we're not turning it off until
"post_disable". It would seem to make sense to move the PLL turning on
to "pre_enable" to match. Unfortunately, I don't believe that's
allowed. It looks as if (in the non-refclk mode which probably nobody
is using) we have to wait until the MIPI clock is there before we can
enable the PLL. In any case, the way it is here won't really
hurt--it'll just leave the PLL on a little longer.

Fixes: a095f15c ("drm/bridge: add support for sn65dsi86 bridge driver")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Robert Foss <robert.foss@linaro.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210730084534.v2.2.If8a8ec3bf1855cf0dbb62c005a71d6698c99c125@changeid