nvfx: restore BEGIN_RING usage

Michel Hermier reported libdrm segfault (and kernel crash) on nv40 using
gallium :
http://www.mail-archive.com/nouveau@lists.freedesktop.org/msg06563.html

It turns out these were caused by some missing WAIT_RING (or wrong
computation of the WAIT_RING sizes). Unlike all other libdrm_nouveau users,
nvfx gallium tried to use a mininum calls of WAIT_RING, one WAIT_RING could
apply to many methods for different code paths and spread across several
functions. This made it too tricky to find out what the missing or wrong
WAIT_RING was.

By restoring BEGIN_RING, we force one WAIT_RING per method, and it's much
easier to check if the free size required in the pushbuffer is correct.  As
curro said, "let's keep it simple for the maintainers until the big
bottlenecks are gone"

Benchmarked on nv35 with openarena, nexuiz and ut2004 and no performance
regression.

The core of this patch was made with Coccinelle, with minor manual fixes
made on top.

Tested-by: Michel Hermier <hermier@frugalware.org>
Signed-off-by: Francisco Jerez <currojerez@riseup.net>

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