Fix the following coccicheck warnings:

./drivers/gpu/drm/amd/display/dc/dce/dmub_psr.c:273:16-21: WARNING:
conversion to bool not needed here.
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>