drm/radeon: Simplify bool comparison

Fix the following coccicheck warning:

./drivers/gpu/drm/radeon/rs690.c:190:6-35: WARNING: Comparison to bool.

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>