iris: Track bound and writable SSBOs

iris: Track bound and writable SSBOs

Marek recently extended pipe->set_shader_buffers() to take an extra
writable_bitmask parameter, indicating which SSBOs are writable (some
may be bound read-only).  We can use this to decide whether to set
EXEC_OBJECT_WRITE when pinning.  Avoiding the write flag can save us
some cross-batch flushing if the SSBO is used for reading in both the
render and compute engines.