drm/amd/display: Ensure index calculation will not overflow

[WHY & HOW]
Make sure vmid0p72_idx, vnom0p8_idx and vmax0p9_idx calculation will
never overflow and exceess array size.

This fixes 3 OVERRUN and 1 INTEGER_OVERFLOW issues reported by Coverity.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>