msm: kgsl: Do not allow CPU mappings of secure buffers

We should not be mapping secure buffers into the CPU pagetables
since these buffers should only be accessed by the GPU in secure
mode. If the userspace does try to access this buffer, that will
trigger a stage 2 page fault and crash the device. This change
was earlier merged in 'commit 926e9ca3 ("msm: kgsl: Do not
allow CPU mappings of secure buffers")' but it was accidently
removed in 'commit a2184922 ("msm: kgsl: Refactor GPU
addressing")'.

Change-Id: I054d34b39e301799c8216a6ffba1099348d8c268
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>
Signed-off-by: Lokesh Batra <lbatra@codeaurora.org>