drm/amd/display: Delete several unneeded bool conversions

The result of an expression consisting of a single relational operator is
already of the bool type and does not need to be evaluated explicitly.

No functional change.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>