drm/amd/display: Simplify bool conversion

Fix the following coccicheck warning:
./drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c:8142:16-21: WARNING:
conversion to bool not needed here

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>