drm/i915/fbdev: fixup setting screen_size

Since we are actually mapping the object and not the vma, when dealing
with LMEM, we should be careful and use the backing store size here,
since the vma->node.size could have all kinds of funny padding
constraints, which could result in us writing to OOB address.

v2(Chris):
  - Prefer vma->size here, which should be the backing store size. Some
    more rework is needed here to stop using node.size in some other
    places.
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220304095934.925036-1-matthew.auld@intel.com

drivers/gpu/drm/i915/display/intel_fbdev.c

@@ -279,7 +279,7 @@ static int intelfb_create(struct drm_fb_helper *helper,
 		/* Our framebuffer is the entirety of fbdev's system memory */
 		info->fix.smem_start =
 			(unsigned long)(ggtt->gmadr.start + vma->node.start);
-		info->fix.smem_len = vma->node.size;
+		info->fix.smem_len = vma->size;
 	}
 
 	vaddr = i915_vma_pin_iomap(vma);
@@ -290,7 +290,7 @@ static int intelfb_create(struct drm_fb_helper *helper,
 		goto out_unpin;
 	}
 	info->screen_base = vaddr;
-	info->screen_size = vma->node.size;
+	info->screen_size = vma->size;
 
 	drm_fb_helper_fill_info(info, &ifbdev->helper, sizes);