drm/amd/display: Fix coding error in connector atomic check

[why]
For MST connector atomic check we have to check a new CRTC state
instead of an old one, when checking if CRTC is disabled to
release VCPI slots allocated.

Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>