drm/i915/gt: Program mocs:63 for cache eviction on gen9

Ville noticed that the last mocs entry is used unconditionally by the HW
when it performs cache evictions, and noted that while the value is not
meant to be writable by the driver, we should program it to a reasonable
value nevertheless.

As it turns out, we can change the value of mocs:63 and the value we
were programming into it would cause hard hangs in conjunction with
atomic operations.

v2: Add details from bspec about how it is used by HW
Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2707
Fixes: 3bbaba0c ("drm/i915: Added Programming of the MOCS")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: <stable@vger.kernel.org> # v4.3+
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201126140841.1982-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_mocs.c

@@ -131,7 +131,19 @@ static const struct drm_i915_mocs_entry skl_mocs_table[] = {
 	GEN9_MOCS_ENTRIES,
 	MOCS_ENTRY(I915_MOCS_CACHED,
 		   LE_3_WB | LE_TC_2_LLC_ELLC | LE_LRUM(3),
-		   L3_3_WB)
+		   L3_3_WB),
+
+	/*
+	 * mocs:63
+	 * - used by the L3 for all of its evictions.
+	 *   Thus it is expected to allow LLC cacheability to enable coherent
+	 *   flows to be maintained.
+	 * - used to force L3 uncachable cycles.
+	 *   Thus it is expected to make the surface L3 uncacheable.
+	 */
+	MOCS_ENTRY(63,
+		   LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
+		   L3_1_UC)
 };
 
 /* NOTE: the LE_TGT_CACHE is not used on Broxton */