st/dri: move TFP code to dri_drawable.c

st/dri: move TFP code to dri_drawable.c

This is based on a patch by nobled <nobled@dreamwidth.org> and allows the TFP
extension to be enabled for DRISW also. This patch does not enable TFP for DRISW
though, because testing on xephyr segfaults here (for both classic and gallium):

Program received signal SIGSEGV, Segmentation fault.
0x00786a4a in _mesa_GenTextures (n=1, textures=0xbfffee4c) at main/texobj.c:788
788	   ASSERT_OUTSIDE_BEGIN_END(ctx);
(gdb)
(gdb) where
\#0  0x00786a4a in _mesa_GenTextures (n=1, textures=0xbfffee4c) at main/texobj.c:788
\#1  0x0817a004 in __glXDisp_GenTextures ()
\#2  0x08168498 in __glXDispatch ()
\#3  0x0808b6ce in Dispatch ()
\#4  0x08084435 in main ()

The TFP code is generic except for the teximage call. We need to verify that
DRISW correclty implements whatever hook teximage finally calls.