backend-drm/state-propose: Don't allow always for updates to the cursor

In some situations, like positioning a sub-surface that exceeds the output's dimensions we would adjust the plane state dimensions to some lower values to that of the buffer. That would ultimately trip the cursor update function because the buffer itself actually exceeds the maximum size/dimension of the cursor.

The plane state destination co-ordinates is area of the view which is visible on the output, which in some situations, would actually be smaller than the original buffer dimensions (making it so that it pass the cropping/scalling check), depending on of how large is the (original) surface (but tripping the assert wrt to cursor width/height dimensions).

This hasn't been seen so far due to the fact that until recently we had a cursor surface that reached the cursor plane and that already being set-up by default (with desktop-shell, which is no longer the case), and also because kiosk-shell doesn't set-up a cursor surface as well.

Signed-off-by: Marius Vlad marius.vlad@collabora.com