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: 97ea6565

 ("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>