drm/amd/display: Fix in disabling secure display

[Why]
Currently we don't check if secure display is enabled before we send
command to disable secure display in dmub. It will accidentally cause
some other igt tests to fail, eg, crtc-linear-degamma.

[How]
Refactor the code we reset the secure display state to check secure
display was enabled or not before stopping it in dmub.

Reviewed-by: Wayne Lin <Wayne.Lin@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alan Liu <HaoPing.Liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>