r600g: initialize CMASK and HTILE with the GPU using streamout

r600g: initialize CMASK and HTILE with the GPU using streamout

This fixes a crash when a resource cannot be mapped to the CPU's address space
because it's too big.

This puts a global pipe_context in r600_screen, which is guarded by a mutex,
so that we can use pipe_context when there isn't one around.
Hopefully our multi-context support is solid.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
NOTE: This is a candidate for the 9.1 branch.