drm/msm: Fix the SNAPSHOT_HEADER macro

The "_header" field of the macro was being incorrectly expanded to just
"header". This was only working because all the functions which used this
macro already had "header" defined in scope.

Change-Id: I19e77ae78cfff471ddffd428cb3fd055c6340737
Signed-off-by: Sharat Masetty <smasetty@codeaurora.org>