drm/amd/display: stop doing unnecessary detection when going to D3

[Why]
Don't want to start HW discovery unless we have lost power,
as doing rediscovery otherwise is both unnecessary and time consuming.
Before this change it takes 40 seconds to go in to suspend, after it
takes 27 seconds

[How]
Accelerated mode gets cleared if we lose power. Only do detection if
this register is cleared

Signed-off-by: Joseph Gravenor <joseph.gravenor@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>