drm/tegra: dc: Use direct offset to plane registers

drm/tegra: dc: Use direct offset to plane registers

Traditionally, windows were accessed indirectly, through a register
selection window that required a global register to be programmed with
the index of the window to access. Since the global register could be
written from modesetting functions as well as the interrupt handler
concurrently, accesses had to be serialized using a lock. Using direct
accesses to the window registers the lock can be avoided.

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