r600g/compute: Defer the creation of the temporary resource

r600g/compute: Defer the creation of the temporary resource

For the first use of a buffer, we will only need the temporary
resource in the case that a user wants to write/map to this buffer.

But in the cases where the user creates a buffer to act as an
output of a kernel, then we were creating an unneeded resource,
because it will contain garbage, and would be copied to the pool,
and destroyed when promoting.

This patch avoids the creation and copies of resources in
this case.

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