drm/vmwgfx: don't check for old_crtc_state enable status

During atomic check to prepare the new topology no need to check if
old_crtc_state was enabled or not. This will cause atomic_check to fail
because due to connector routing a crtc can be in atomic_state even if
there was no change to enable status.

Detected this issue with igt run.

Signed-off-by: Deepak Rawat <drawat@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>