drm/amd/display: correct idle_power_optimizations disablement return value

[Why]
The return value indicates whether the operation(disable/enable) succeeded
or not. The existing logic reports wrong result even if the disablement was
performed successfully. That will make succeeding reenablement abandoned
as dc->idle_optimizations_allowed is always true.

[How]
Correct the return value to reflect the real result of disablement.

Fixes: 235c6763 ("drm/amd/display: add DCN32/321 specific files for Display Core")
Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>