drm/amd/display: fix calculation for the pwl backlight curve

[Why]
The PWL backlight curve is used by the firmware to convert between
brightness and linear PWM value.
Driver has a backlight LUT, but the firmware holds a PWL curve and
interpolates between points.

The calculations are incorrect leading to slightly off backlight values
being programmed.

[How]
Fix the PWL backlight curve threshold/offset calculations

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Josip Pavic <Josip.Pavic@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>