Fix SiS AGP vertex dispatch by not trying to emit 0 vertices, which would hang

the hardware.  Re-enable AGP by default.

src/mesa/drivers/dri/sis/sis_screen.c

@@ -46,7 +46,7 @@ CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 const char __driConfigOptions[] =
 DRI_CONF_BEGIN
 	DRI_CONF_SECTION_DEBUG
-		DRI_CONF_OPT_BEGIN(agp_disable,bool,true)
+		DRI_CONF_OPT_BEGIN(agp_disable,bool,false)
 		DRI_CONF_DESC(en,"Disable AGP vertex dispatch")
 		DRI_CONF_OPT_END
 		DRI_CONF_OPT_BEGIN(fallback_force,bool,false)

src/mesa/drivers/dri/sis/sis_tris.c

@@ -889,7 +889,8 @@ static void sisRenderFinish( GLcontext *ctx )
 void
 sisFlushPrimsLocked(sisContextPtr smesa)
 {
-   GLuint *start;
+   if (smesa->vb_cur == smesa->vb_last)
+      return;
 
    sisUpdateHWState(smesa->glCtx);
 
@@ -898,7 +899,7 @@ sisFlushPrimsLocked(sisContextPtr smesa)
       mEndPrimitive();
       MMIO(REG_3D_AGPCmBase, (smesa->vb_last - smesa->vb) +
          smesa->vb_agp_offset);
-      MMIO(REG_3D_AGPTtDwNum, (smesa->vb_cur - smesa->vb_last) / 4 |
+      MMIO(REG_3D_AGPTtDwNum, ((smesa->vb_cur - smesa->vb_last) / 4) |
 	 0x50000000);
       MMIO(REG_3D_ParsingSet, smesa->AGPParseSet);
       MMIO(REG_3D_AGPCmFire, (GLint)(-1));

src/mesa/drivers/dri/sis/sis_tris.h

@@ -51,6 +51,7 @@ do {							\
 static __inline GLuint *sisAllocDmaLow(sisContextPtr smesa, int bytes)
 {
    GLuint *start;
+
    if (smesa->vb_cur + bytes >= smesa->vb_end) {
       LOCK_HARDWARE();
       sisFlushPrimsLocked(smesa);