Remove explicit locking and kmap arguments from GEM VRAM interface

[Thomas Zimmermann]

Drivers should not have to care about internal locking of GEM VRAM objects
and their memory-mapping structures. This patch set removes both from the
GEM VRAM interface.

This affects the ast and mgag200 drivers. In places where GEM objects are
being locked by the driver, the patch converts the lock operation to a pin
operation. The locking prevented the memory manager from moving the object,
so pinning is more appropriate.

For the memory mappings, all book-keeping is done by the implementation
of GEM VRAM. Explicit kmap objects are removed from the module's public
interfaces. This change mostly affects the cursor handling in ast and
mgag200, which is being simplified by this patch set.

Future directions: with these patches in place, more code in mode setting
and fbdev emulation can be shared between ast and mgag200.

The patches have been tested on ast and mgag200 hardware.