drm/amd/display: Handle interpolation for first data point

drm/amd/display: Handle interpolation for first data point

[Why]
If the first data point for a custom brightness curve is not 0% luminance
then the first few luminance values will be ignored.

[How]
Check signal is below first data point and if so do linear interpolation to
0 instead.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Ivan Lipski <ivan.lipski@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>