drm/amd/display: Move enable fractional pwm call

[Why]
Dmcu init fw call has some logic to initialize abm values.  Since this
doesn't exist on dmcub, must find a proper place for it in the abm
sequence.

[How]
Move enable fractional pwm call.

Signed-off-by: Wyatt Wood <wyatt.wood@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>