drm/amd/display: remove clearance code of force_ffu_mode flag in dmub_psr_copy_settings()

[Why/How]
The force_ffu_mode flag could be initialized at other place.

Reviewed-by: Robin Chen <robin.chen@amd.com>
Signed-off-by: Zhongwei <Zhongwei.Zhang@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>