Enable DRI3 by default when building for Xorg >= 1.18.3

Seems to work well enough in general now.

(Ported from radeon commit 1181b9c582f10b6c523e4b2988e2ce87ecf3d367)

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>

man/amdgpu.man

@@ -64,6 +64,8 @@ will assign xrandr outputs LVDS and VGA-0 to this instance of the driver.
 .BI "Option \*qDRI\*q \*q" integer \*q
 Define the maximum level of DRI to enable. Valid values are 2 for DRI2 or 3 for DRI3.
 The default is
+.B 3 for DRI3
+if the driver was compiled for Xorg >= 1.18.3, otherwise
 .B 2 for DRI2.
 .TP
 .BI "Option \*qEnablePageFlip\*q \*q" boolean \*q

src/amdgpu_kms.c

@@ -1230,7 +1230,11 @@ Bool AMDGPUScreenInit_KMS(SCREEN_INIT_ARGS_DECL)
 	}
 #endif
 
+#if XORG_VERSION_CURRENT >= XORG_VERSION_NUMERIC(1,18,3,0,0)
+	value = TRUE;
+#else
 	value = FALSE;
+#endif
 	from = X_DEFAULT;
 
 	if (info->use_glamor) {