drm/i915/gvt: IS_ERR() vs NULL bug in intel_gvt_update_reg_whitelist()

The shmem_pin_map() function returns NULL, it doesn't return error
pointers.

Fixes: 97ea656 ("drm/i915/gvt: Parse default state to update reg whitelist")
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/Ysftoia2BPUyqVcD@kili
Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>

Author: Dan Carpenter

drivers/gpu/drm/i915/gvt/cmd_parser.c

@@ -3115,9 +3115,9 @@ void intel_gvt_update_reg_whitelist(struct intel_vgpu *vgpu)
 			continue;
 
 		vaddr = shmem_pin_map(engine->default_state);
-		if (IS_ERR(vaddr)) {
-			gvt_err("failed to map %s->default state, err:%zd\n",
-				engine->name, PTR_ERR(vaddr));
+		if (!vaddr) {
+			gvt_err("failed to map %s->default state\n",
+				engine->name);
 			return;
 		}