drm/amd/display: Drop amdgpu_dm_prev_state struct

[Why]
It's not being used

[How]
Nuke it

Signed-off-by: Leo Li <sunpeng.li@amd.com>
Reviewed-by: David Francis <David.Francis@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>