drm/amd/display: Add switch for Fractional PWM on or off

[Why]
Some LED Driver might not like Fractional PWM especially at extreme
ranges near 0% or 100%.
For example, backlight flashing could be observed.
We want a way to switch fractional PWM on/off either for debug, or
possibly production.

[How]
Add DC code that can send new FW command to enable/disable
fractional PWM.

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>