i965: Upload invariant state once at the start of the batch on Gen4-5.

We want to emit invariant state at the start of a render batch.  In the
past, this more or less happened: a new batch flagged BRW_NEW_CONTEXT
(because we don't have hardware contexts), which triggered the
brw_invariant_state atom.  So, it would be emitted before any 3D
drawing.  (Technically, there might be some BLT commands in the batch
because Gen4-5 have a single combined render/BLT ring, but that should
be harmless).

With the advent of BLORP, this broke.  The first item in a batch might
be a BLORP operation, which bypasses the normal draw upload path.  So,
we need to ensure invariant state happens first.  To do that, we just
upload it when creating a new batch.  On Gen6+ we'd need to worry about
whether it's a RENDER or BLT batch, but because we have a combined ring,
this approach should work fine on Gen4-5.

Seems to fix GPU hangs when playing hardware accelerated video with
mpv -hwdec=vaapi on Ironlake.

Cc: mesa-stable@lists.freedesktop.org
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103529


Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit 8f91aa35)