staging: etnaviv: publish and use mmu geometry

We model the GPU MMU using the iommu layer, which supports exporting
the iommu domain geometry.  Use this feature to publish the size of
the MMU window, and initialise the MMU drm_mm object according to
the available MMU window size.

As we only allocate a MMU page table which covers 256MB, yet we
initialised the drm_mm object to cover 1GB, this fixes an overflow
of the MMU page table array.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>