ps3fb: GPU memory mapping cleanup

- Make the IOMMU flags used for mapping main memory into the GPU's I/O space
    explicit, instead of relying on the default in the hypervisor,
  - Add missing calls to lv1_gpu_context_iomap(..., CBE_IOPTE_M) to unmap the
    memory during cleanup.

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Cc: linux-fbdev-devel@lists.sourceforge.net
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>