render/allocator/drm_dumb: advertise LINEAR instead of INVALID modifier

INVALID means that the modifier is implicit. However dumb buffers
are guaranteed to be LINEAR, so let's just advertise this. Fixes
cursors with the DRM backend: cursor planes usually only support
LINEAR.

render/allocator/drm_dumb.c

@@ -111,7 +111,7 @@ static struct wlr_drm_dumb_buffer *create_buffer(
 		.width = buffer->width,
 		.height = buffer->height,
 		.format = format->format,
-		.modifier = DRM_FORMAT_MOD_INVALID,
+		.modifier = DRM_FORMAT_MOD_LINEAR,
 		.n_planes = 1,
 		.offset[0] = 0,
 		.stride[0] = buffer->stride,