radv,gallium: Add driconf option to reduce advertised VRAM size.

To help debugging games that actually do active memory budget
management.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6641>

src/util/driconf.h

@@ -222,6 +222,11 @@ DRI_CONF_OPT_BEGIN_B(force_compat_profile, def) \
         DRI_CONF_DESC("Force an OpenGL compatibility context") \
 DRI_CONF_OPT_END
 
+#define DRI_CONF_OVERRIDE_VRAM_SIZE() \
+DRI_CONF_OPT_BEGIN_V(override_vram_size, int, -1, "-1:2147483647") \
+        DRI_CONF_DESC("Override the VRAM size advertised to the application in MiB (-1 = default)") \
+DRI_CONF_OPT_END
+
 /**
  * \brief Image quality-related options
  */