drm/amd/pm: remove useless if

Clean the following coccicheck warning:

./drivers/gpu/drm/amd/pm/legacy-dpm/si_dpm.c:7035:2-4: WARNING: possible
condition with no effect (if == else).

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>