drm:amdgpu:remove unneeded variable

return value form directly instead of
taking this in another redundant variable.

Reported-by: Zeal Robot <zealci@zte.com.cm>
Signed-off-by: chiminghao <chi.minghao@zte.com.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>