Turns out both kernel v5.10 and v5.11 have the same amdgpu driver
version and only one has modifiers ... In addition the version check
is kinda annoying for backports.

So lets use the cap. Since the cap is technically about ADDFB2 I
tested that this works on rendernodes (and reading the code there
is no distinction from what kind of node this is called).

Fixes: 9a937330

 ("radeonsi: Only set modifier creation function for GFX9+ & with kernel support.")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <!10337>