This reverts commit cfeff354.

A core design consideration with legacy cursor updates is that the
cursor must not touch any other plane, even if we were to force it
to take the slow path. That is the real reason why the cursor uses
a fixed ddb allocation, not because bspec says so.

Treating cursors as any other plane during ddb allocation
violates that, which means we can now pull other planes into
fully unsynced legacy cursor mailbox commits. That is
definitely not something we've ever considered when designing
the rest of the code. The noarm+arm register write split in
particular makes that dangerous as previous updates can get
disarmed pretty much at any random time, and not necessarily
in an order that is actually safe (eg. against ddb overlaps).

So if we were to do this then:
- someone needs to expend the appropriate amount of brain
  cells thinking through all the tricky details
- we should do it for all skl+ platforms since all
  of those have double buffered wm/ddb registers. The current
  arbitrary mtl+ cutoff doesn't really make sense

For the moment just go back to the original behaviour where
the cursor's ddb alloation does not change outside of
modeset/fastset. As of now anything else isn't safe.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231213102519.13500-10-ville.syrjala@linux.intel.comReviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>