drm/i915: Disable snooping (userptr, set-cache-level) on gen4

The original gen4 has an issue where writes (both render and blt) into
snoopable pages are lost. We've previously worked around this in
userspace (ddx, igt) by simply not requesting snoopable buffers, but upon
rediscovering this problem for a third time, make the kernel reject such
requests with -ENODEV.

This disables snooping on userspace buffers for i965g and i965gm (original
gen4) machines.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170906192424.26970-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_pci.c

@@ -168,6 +168,7 @@ static const struct intel_device_info intel_i965g_info __initconst = {
 	.platform = INTEL_I965G,
 	.has_overlay = 1,
 	.hws_needs_physical = 1,
+	.has_snoop = false,
 };
 
 static const struct intel_device_info intel_i965gm_info __initconst = {
@@ -177,6 +178,7 @@ static const struct intel_device_info intel_i965gm_info __initconst = {
 	.has_overlay = 1,
 	.supports_tv = 1,
 	.hws_needs_physical = 1,
+	.has_snoop = false,
 };
 
 static const struct intel_device_info intel_g45_info __initconst = {

drivers/gpu/drm/i915/intel_device_info.c

@@ -412,8 +412,6 @@ void intel_device_info_runtime_init(struct drm_i915_private *dev_priv)
 	else if (INTEL_INFO(dev_priv)->gen >= 9)
 		gen9_sseu_info_init(dev_priv);
 
-	WARN_ON(info->has_snoop != !info->has_llc);
-
 	DRM_DEBUG_DRIVER("slice mask: %04x\n", info->sseu.slice_mask);
 	DRM_DEBUG_DRIVER("slice total: %u\n", hweight8(info->sseu.slice_mask));
 	DRM_DEBUG_DRIVER("subslice total: %u\n",