drm/amd/display: Add definition for number of backlight data points

[Why]
A hardcoded number is used today

[How]
Add definition for number of BL data points

Signed-off-by: Camille Cho <Camille.Cho@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>