drm/amd/display: wrong VM setting cause invalid DF request from DCN

fix typo in register field.  we are lucky the shift/mask is the same, no behavior change

add globals to experiment with using different VM settings

Signed-off-by: Tony Cheng <tony.cheng@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>