drm/amd/display: Clean up old pplib interface functions

[Why]
set_display_requirement, dcn1_pplib_apply_display_requirements
are no longer used and should be removed.

Signed-off-by: Fatemeh Darbehani <fatemeh.darbehani@amd.com>
Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>