drm/tegra: dc: Select root window for event dispatch

In finish pageflip, the driver was not selecting the root window when
dispatching events. This exposed a race where a plane update would
change the window selection and cause tegra_dc_finish_page_flip to check
the wrong base address.

This patch also protects access to the window selection register as well
as the registers affected by it.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>