drm/amd/display: Fix OTG H timing reset for dcn314

[Why]
When ODM is enabled, H timing control register reset
to 0. Div mode manual field get overwritten causing
no display on certain modes for dcn314.

[How]
Use REG_UPDATE instead of REG_SET to set div_mode
field.

Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Brian Chang <Brian.Chang@amd.com>
Signed-off-by: Duncan Ma <duncan.ma@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>