backend/drm: force LINEAR for cursor plane formats
backend/drm: force LINEAR for cursor plane formats

If the kernel driver doesn't support modifiers, it still expects cursor
FBs to have a LINEAR layout. See [1] for expectations for framebuffers
attached to the cursor plane.

[1]: https://patchwork.freedesktop.org/patch/408512/

swaywm/wlroots,SirCmpwn/wlroots,swaywm/wlroots