msm: kgsl: Remove VM_MAYWRITE flag to restrict mprotect

When VM_MAYWRITE flag is used during mmap(), mprotect()
can be used later to change the protection of memstore
to allow write. Make sure this does not happen by
removing VM_MAYWRITE from the vm_flags of vma.

Change-Id: I6f69f05858ea40611d512cfa796caabeaa88cdb5
Signed-off-by: Indira Biruduraju <ibirudur@codeaurora.org>