drm/i915: In the debugfs interface, unmap our address instead of the page's.

Fixes a BUG_ON in kmap_atomic for the following atomic mapping with
USER0 type.
Signed-off-by: NEric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/i915_debugfs.c

@@ -272,7 +272,7 @@ static void i915_dump_pages(struct seq_file *m, struct page **pages, int page_co
 		mem = kmap_atomic(pages[page], KM_USER0);
 		for (i = 0; i < PAGE_SIZE; i += 4)
 			seq_printf(m, "%08x :  %08x\n", i, mem[i / 4]);
-		kunmap_atomic(pages[page], KM_USER0);
+		kunmap_atomic(mem, KM_USER0);
 	}
 }