drm/amd/display: fix typos for dcn20_funcs and dcn21_funcs struct

In dcn20_funcs and dcn21_funcs struct, the member ".dsc_pg_control = NULL"
should be removed due to .dsc_pg_control be assigned to dcn20_dsc_pg_control.

Signed-off-by: Stanley.Yang <Stanley.Yang@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>