msm: kgsl: Fix race condition while making page as dirty

set_page_dirty() is racy if the caller has no
reference against page->mapping->host, and if
the page is unlocked. This is because another
CPU could truncate the page off the mapping and
then free the mapping.

Use set_page_dirty_lock() to avoid this race condition.

Change-Id: I517fb9aee66560618c7676b311368f7a7498011f
Signed-off-by: Rajesh Kemisetti <rajeshk@codeaurora.org>
Signed-off-by: Archana Sriram <apsrir@codeaurora.org>