drm/tegra: dc: Unify enabling the display controller

Previously output drivers would enable continuous display mode and power
up the display controller at various points during the initialization.
This is suboptimal because it accesses display controller registers in
output drivers and duplicates a bit of code.

Move this code into the display controller driver and enable the display
controller as the final step of the ->mode_set_nofb() implementation.

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