staging: etnaviv: move GPU memory management into MMU

The GPU memory management (managed by a drm_mm object) is used to
track which areas of the MMU address space are in-use.  Therefore,
this should be tied to the MMU object, rather than the GPU object.

This means we could (as comments suggest) have multiple MMU objects,
one for each context, and switch between them.  Each would need to
be managed by its own drm_mm object.

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