drm/amd/display: Remove useless else if

The assignment of the else and if branches is the same, so the if else
here is redundant, so we remove it.

./drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c:1951:2-4: WARNING: possible condition with no effect (if == else).

Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=3719


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