r600g/compute: Avoid problems when promoting items mapped for reading

r600g/compute: Avoid problems when promoting items mapped for reading

Acording to the OpenCL spec, it is possible to have a buffer mapped
for reading and at read from it using commands or buffers.

With this we can keep the mapping (that exists against the
temporary item) and read with a kernel (from the item we have
just added to the pool) without problems.

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>