plugins: avoid log flood when activating pool

plugins: avoid log flood when activating pool

Every time a new buffer is allocated, the pool is activated. This
doesn't impact in performance since gst_buffer_pool_set_active()
checks the current state of the pool. Nonetheless it logs out a
message if the state is the same, and it floods the logging subsystem
if it is enabled.

To avoid this log flooding first the pool state is checked before
changing it.