drm/i915/skl: Fix up positive error code

It should have been negative since it is returned with ERR_PTR().

Introduced in new code commit:

   commit 50470bb0
   Author: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
   Date:   Mon Mar 23 11:10:36 2015 +0000

    drm/i915/skl: Support secondary (rotated) frame buffer mapping
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -2540,7 +2540,7 @@ intel_rotate_fb_obj_pages(struct i915_ggtt_view *ggtt_view,
 	struct sg_table *st;
 	unsigned int tile_pitch, tile_height;
 	unsigned int width_pages, height_pages;
-	int ret = ENOMEM;
+	int ret = -ENOMEM;
 
 	pages = obj->base.size / PAGE_SIZE;