drm/amd/display: fix check condition for edp power control

Per discussion with VBIOS team, the orginal check is not correct in
all cases on latest VBIOS. Additional check is needed. This change should
maintain old behaviour on older VBIOS.

Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>