drm/amd/display: Handle persistence in DM

[Why]
Remove dm_write_persistent_data and dm_read_persistent_data as
persistence should be handled in DM.

[How]
Remove functions. Move read/write calls into DM layer while maintaining
logic.

Signed-off-by: Jaehyun Chung <jaehyun.chung@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>