drm/tegra: dc: Add powergate support

Both display controllers are in their own power partition. Currently the
driver relies on the assumption that these partitions are on (which is
the hardware default). However some bootloaders may disable them, so the
driver must make sure to turn them back on to avoid hangs.

Signed-off-by: Thierry Reding <treding@nvidia.com>

diff --cc drivers/gpu/drm/tegra/dc.c
Simple merge

diff --cc drivers/gpu/drm/tegra/drm.h
Simple merge