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: be1cb55a07bf ("drm/i915/gt: Keep a no-frills swappable copy of the default context state")
Signed-off-by: Dan Carpenter <[e-mail]>
Reviewed-by: Matthew Auld <[e-mail]>
Signed-off-by: Matthew Auld <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/20220708094104.GL2316@kadam

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

···
 			continue;
 
 		hw = shmem_pin_map(engine->default_state);
-		if (IS_ERR(hw)) {
-			err = PTR_ERR(hw);
+		if (!hw) {
+			err = -ENOMEM;
 			break;
 		}
 		hw += LRC_STATE_OFFSET / sizeof(*hw);
···
 			continue;
 
 		hw = shmem_pin_map(engine->default_state);
-		if (IS_ERR(hw)) {
-			err = PTR_ERR(hw);
+		if (!hw) {
+			err = -ENOMEM;
 			break;
 		}
 		hw += LRC_STATE_OFFSET / sizeof(*hw);