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.

Change-Id: I4464a0a4edbc33df155b6c4103e77fc81d02e84a
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>