drm/amd/display: fix dp_ss_control vbios flag parsing

dp_ss_control = 0 means ss is off, we had a typo where
we would double not dp_ss_control while setting dp_ss_off
flag

Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Reviewed-by: Eric Bernstein <Eric.Bernstein@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>