r300: Disable threshold register for KMS.

Kernel side doesn't have required register in safe list so can't send it kernel.

src/mesa/drivers/dri/r300/r300_cmdbuf.c

@@ -685,9 +685,10 @@ void r300InitCmdBuf(r300ContextPtr r300)
 	ALLOC_STATE(rb3d_aaresolve_ctl, always, 2, 0);
 	r300->hw.rb3d_aaresolve_ctl.cmd[0] = cmdpacket0(r300->radeon.radeonScreen, R300_RB3D_AARESOLVE_CTL, 1);
 	if ((r300->radeon.radeonScreen->chip_family >= CHIP_FAMILY_RV515) ||
+	      ( !r300->radeon.radeonScreen->kernel_mm && (
 	    (r300->radeon.radeonScreen->chip_family == CHIP_FAMILY_RS400) ||
 	    (r300->radeon.radeonScreen->chip_family == CHIP_FAMILY_RV410) ||
-	    (r300->radeon.radeonScreen->chip_family == CHIP_FAMILY_R420)) {
+	    (r300->radeon.radeonScreen->chip_family == CHIP_FAMILY_R420) ) ) ) {
 		ALLOC_STATE(rb3d_discard_src_pixel_lte_threshold, always, 3, 0);
 	} else {
 		ALLOC_STATE(rb3d_discard_src_pixel_lte_threshold, never, 3, 0);