i965: Work around mysterious Gen4 GPU hangs with minimal state changes.

i965: Work around mysterious Gen4 GPU hangs with minimal state changes.

Gen4 hardware appears to GPU hang frequently when using Chromium, and
also when running 'glmark2 -b ideas'.  Most of the error states contain
3DPRIMITIVE commands in quick succession, with very few state packets
between them - usually VERTEX_BUFFERS/ELEMENTS and CONSTANT_BUFFER.

I trimmed an apitrace of the glmark2 hang down to two draw calls with a
glUniformMatrix4fv call between the two.  Either draw by itself works
fine, but together, they hang the GPU.  Removing the glUniform call
makes the hangs disappear.  In the hardware state, this translates to
removing the CONSTANT_BUFFER packet between the two 3DPRIMITIVE packets.

Flushing before emitting CONSTANT_BUFFER packets also appears to make
the hangs disappear.  I observed a slowdown in glxgears by doing it all
the time, so I've chosen to only do it when BRW_NEW_BATCH and
BRW_NEW_PSP are unset (i.e. we haven't done a CS_URB_STATE change or
already flushed the whole pipeline).

I'd much rather understand the problem, but at this point, I don't see
how we'd ever be able to track it down further.  We have no real tools,
and the hardware people moved on years ago.  I've analyzed 20+ error
states and read every scrap of documentation I could find.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=80568
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=85367
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Matt Turner <mattst88@gmail.com>
Cc: "10.4 10.3" <mesa-stable@lists.freedesktop.org>