drm/amd/display: Remove unused function reverse_planes_order

The function are defined in the amdgpu_dm.c file, but not called
anywhere, so delete the unused function.

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:371:20: warning: unused function 'reverse_planes_order'.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=9402


Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>