drm/i915/selftests: fix a couple IS_ERR() vs NULL tests

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

Fixes: be1cb55a

 ("drm/i915/gt: Keep a no-frills swappable copy of the default context state")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220708094104.GL2316@kadam