drm/amd/display: Correctly respond in psr enablement interface

[Why]
dc_link_set_psr_allow_active() always returns true, even in the case
that PSR is not supported.

[How]
Hook up the return value of dc_link_set_psr_allow_active().

Signed-off-by: Camille Cho <Camille.Cho@amd.com>
Reviewed-by: Josip Pavic <Josip.Pavic@amd.com>
Acked-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>