mali: Fix VM_MAY% issues for mmap

Trace buffer is inadequately protected from modification by user space.
In addition all other mmap handling only checks for VM_% flags, but
don't clear out the VM_MAY% flags, allowing a client to upgrade their
mapping to more privileges than the driver expected.

This fix clears out the VM_MAY% flags for all mmaps, plus hard-codes the
right flags for the special cases.

Bug: 13539903
Change-Id: Ib5db0e44a752649e662c9295bdc444640c1cb912