drm/amd/display: remove min reduction for abm 2.2 level 3

[Why]
Image brightness compensation for solid color full screen images is
expected to be optimal for ABM 2.2 at level 3. The min reduction that is
currently being enforced prevents this from being achieved.

[How]
Remove the min reduction for ABM 2.2 at level 3

Signed-off-by: Josip Pavic <Josip.Pavic@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>