r600g: delay mapping until first map request. (v2)

r600g: delay mapping until first map request. (v2)

Currently r600g always maps every bo, this is quite pointless as it wastes
VM and on 32-bit with wine running VM space is quite useful.

So with this patch we don't create the mappings until first use, without
tiling enabled this probably won't make a major difference on its own,
but with tiled staged uploads it should avoid keeping maps for most of the
textures unnecessarily.

v2: add bo data ptr check

Signed-off-by: Dave Airlie <airlied@redhat.com>