Before shutting down the display using the DCS display_off command, wait
for 4 frames according to the datasheet.

Furthermore, after enabling the power supply, the supply voltage needs
around 10 ms to settle. After that, another 120 ms is required before a
DCS exit_sleep_mode command can be sent.

While at it, no longer send the DCS soft_reset command. This is totally
unnecessary because we've just powered up the display, hence it will be
in a reset state already.
Signed-off-by: Thierry Reding <treding@nvidia.com>