msm: kgsl: flush cache by virtual address

dmac_flush_range expects the virtual address of the memory to be flushed
but we're passing the physical address. Fix this.

This also fixes the following warning during LPAE builds:

    kernel/drivers/gpu/msm/kgsl_sharedmem.c: In function \
        ‘kgsl_cma_alloc_secure’:
    kernel/drivers/gpu/msm/kgsl_sharedmem.c:940:19: warning: cast to \
        pointer from integer of different size [-Wint-to-pointer-cast]
    error, forbidden warning: kgsl_sharedmem.c:940

Change-Id: I3881ff5d3388bf42476aa9f1e0ff2428e1ef94d7
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>