msm: kgsl: Avoid cache ops on secure memory

There is no check before cache ops if the memory is marked secure.
This leads to stage 2 pagefault if a secure memory is passed to
IOCTL_KGSL_GPUMEM_SYNC_CACHE ioctl because kernel is not allowed to
do cache ops on secure memory. This can be avoided by returning
success immediately if the memory is marked as secure.

Change-Id: I215d77d2a488cdb00e8e18cfd38cddd9632fd9f6
Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>