drivers: uc81xx: Avoid using auto sequencing

The UC8179 supports automatic power management where the DC-DC is
enabled automatically. This is not supported on the UC8176. Explicitly
call PON/DRF/POF from uc81xx_update_display instead. This ensures that
the DC-DC is only enabled while actually updating the display for all
supported chips.

Signed-off-by: Andreas Sandberg <andreas@sandberg.uk>