drm/amd/display: Enable 3 plane for DCN 3.0 and 3.02

[WHY&HOW]
Increase num of bottom planes to 2. Note that DCN 3.03 is left out
since it has max 2 planes.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>