drm/amd/display: Correct MPC split policy for DCN301

[Why]
DCN301 has seamless boot enabled. With MPC split enabled
at the same time, system will hang.

[How]
Revert MPC split policy back to "MPC_SPLIT_AVOID". Since we have
ODM combine enabled on DCN301, pipe split is not necessary here.

Signed-off-by: Zhan Liu <zhan.liu@amd.com>
Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>