drm/amd/display: Validate hw_points_num before using it

[WHAT]
hw_points_num is 0 before ogam LUT is programmed; however, function
"dwb3_program_ogam_pwl" assumes hw_points_num is always greater than 0,
i.e. substracting it by 1 as an array index.

[HOW]
Check hw_points_num is not equal to 0 before using it.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>