pan/bi: Add a define for the Bifrost shader prefetch size

Found by adding NOPs to the start of a shader and checking dmesg to
see at what sizes the GPU faulted trying to read the following
non-executable page.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8428>

src/panfrost/bifrost/bifrost_compile.c

@@ -47,6 +47,10 @@ static const struct debug_named_value bifrost_debug_options[] = {
 
 DEBUG_GET_ONCE_FLAGS_OPTION(bifrost_debug, "BIFROST_MESA_DEBUG", bifrost_debug_options, 0)
 
+/* How many bytes are prefetched by the Bifrost shader core. Past the end of
+ * the shader, this range must contain valid instructions or zero. */
+#define BIFROST_SHADER_PREFETCH 96
+
 /* TODO: This is not thread safe!! */
 static unsigned SHADER_DB_COUNT = 0;