In 93cd3a79b2 ("backend/drm: stop using GBM flags"), we stopped
using the GBM_BO_USE_LINEAR flag in favor of a modifier list set
to { DRM_FORMAT_MOD_LINEAR }. However, the last argument of
drm_plane_init_surface disables modifiers -- so the buffer will just
get allocated with an implicit modifier, without necessarily being
LINEAR.
To fix this, allow modifiers when allocating the cursor buffers.
wlr_drm_plane.formats should already have the necessary LINEAR
restrictions.
Fixes: 93cd3a79b2 ("backend/drm: stop using GBM flags")