drm/amd/display: DSC policy override when ODM combine is forced

[why]
When we force ODM combine with DSC, we lose several
8 bit and 10 bit modes in validation and thus
not able to use HDR. This is due to the number of
horizontal slices used in DSC not properly being
accounted for currently when 2:1 ODM Combine is forced.

[how]
Enforce at least two horizontal slices are used for DSC when
ODM combine is forced.

Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Nasir Osman <nasir.osman@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>