drm/amdgpu/display: set vblank_disable_immediate for DC

Disable vblanks immediately to save power.  I think this was
missed when we merged DC support.

Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1781
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>