i965: Add an INTEL_DEBUG=submit option for printing batch statistics.

i965: Add an INTEL_DEBUG=submit option for printing batch statistics.

When a batch is submitted, INTEL_DEBUG=bat prints a message indicating
which part of the code triggered the flush, and some statistics about
the batch/state buffer utilization.

It also decodes the batchbuffer in debug builds...which is so much
output that it drowns out the utilization messages, if that's all you
care about.

INTEL_DEBUG=submit now just does the utilization messages.
INTEL_DEBUG=bat continues to do both (as the message is a good indicator
that we're starting decode of a new batch).

v2: Rename from "flush" to "submit" (suggested by Chris) because we
    might want "flush" for PIPE_CONTROL debugging someday.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>