r300g: drop begin_cs/end_cs

r300g: drop begin_cs/end_cs

I have had a look at the libdrm sources and they just contain more or less
the same checking we do in macros, and begin_cs may realloc the CS buffer
if we overflow it, which never happens with r300g. So these are pretty
much useless.

There is a small but measurable performance increase by dropping the two
functions.