drm/amdgpu/display: Remove unnecessary conversion to bool

Fix the following coccicheck warnings:

./drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c:956:52-57: WARNING:
conversion to bool not needed here.

./drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c:8311:16-21: 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>