drm/amd/display: fix accidental casting enum to bool

[why/how]
Fixing -Wint-in-bool-context Clang Build Failure

Reviewed-by: Nicholas Choi <Nicholas.Choi@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>