drm/panel: boe-tv101wum-nl6: Ensure DSI writes succeed during disable

The unprepare sequence has started to fail after moving to panel bridge
code in the msm drm driver (commit 007ac0262b0d ("drm/msm/dsi: switch to
DRM_PANEL_BRIDGE")). You'll see messages like this in the kernel logs:

   panel-boe-tv101wum-nl6 ae94000.dsi.0: failed to set panel off: -22

This is because boe_panel_enter_sleep_mode() needs an operating DSI link
to set the panel into sleep mode. Performing those writes in the
unprepare phase of bridge ops is too late, because the link has already
been torn down by the DSI controller in post_disable, i.e. the PHY has
been disabled, etc. See dsi_mgr_bridge_post_disable() for more details
on the DSI .

Split the unprepare function into a disable part and an unprepare part.
For now, just the DSI writes to enter sleep mode are put in the disable
function. This fixes the panel off routine and keeps the panel happy.

My Wormdingler has an integrated touchscreen that stops responding to
touch if the panel is only half disabled too. This patch fixes it. And
finally, this saves power when the screen is off because without this
fix the regulators for the panel are left enabled when nothing is being
displayed on the screen.

Fixes: 007ac0262b0d ("drm/msm/dsi: switch to DRM_PANEL_BRIDGE")
Fixes: a869b9db7adf ("drm/panel: support for boe tv101wum-nl6 wuxga dsi video mode panel")
Cc: yangcong <[email protected]>
Cc: Douglas Anderson <[email protected]>
Cc: Jitao Shi <[email protected]>
Cc: Sam Ravnborg <[email protected]>
Cc: Rob Clark <[email protected]>
Cc: Dmitry Baryshkov <[email protected]>
Signed-off-by: Stephen Boyd <[email protected]>
Reviewed-by: Douglas Anderson <[email protected]>
Signed-off-by: Douglas Anderson <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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