multi-gpu: do not flip screens on secondary GPU

All screens on secondary GPU in multiple GPU configurations
was flipped 180.

The flipped screens was always on secondary card (the primary card
was always correct).

Tested on nouveau with:
WLR_DRM_DEVICES=/dev/dri/card1:/dev/dri/card2
WLR_DRM_DEVICES=/dev/dri/card2:/dev/dri/card1

The commit is fixing this problem. Now all screens are "normal".

backend/drm/renderer.c

@@ -238,7 +238,7 @@ struct gbm_bo *copy_drm_surface_mgpu(struct wlr_drm_surface *dest,
 	assert(tex);
 
 	float mat[9];
-	wlr_matrix_projection(mat, 1, 1, WL_OUTPUT_TRANSFORM_FLIPPED_180);
+	wlr_matrix_projection(mat, 1, 1, WL_OUTPUT_TRANSFORM_NORMAL);
 
 	struct wlr_renderer *renderer = dest->renderer->wlr_rend;
 	wlr_renderer_begin(renderer, dest->width, dest->height);