msm: sde: change rotator BGRA4444 to unique fourcc code

BGRA4444, defined currently as fourcc "BA12" in rotator driver,
is already assigned to existing format V4L2_PIX_FMT_SGRBG12 as
defined in videodev2.h.  Change BGRA4444 to new fourcc definition
"bA12" to avoid collision.

CRs-Fixed: 1054825
Change-Id: I8b28dd3827cc9b540eef5955bbf9dbe77ce91f4b
Signed-off-by: Alan Kwong <akwong@codeaurora.org>