msm: kgsl: do not set private flag for secure allocations

The intention to set page private flag was to avoid
importing secure buffers as non secure buffers. This would
result in secure buffers being incorrectly mapped to a non
secure context bank. This problem can arise when importing
ion buffers, but never with a secure allocation from kgsl.
There is no means to import a secure kgsl buffer as
non secure buffer. So setting page private for secure
allocations done by kgsl can be safely removed.

Change-Id: I9ea879ec36f945a0f1fd57055dc7d4fa3bfe8905
Signed-off-by: Puranam V G Tejaswi <pvgtejas@codeaurora.org>