drm/amd/amdkfd: Drop unnecessary NULL check after container_of

The first parameter passed to container_of() is the pointer to the work
structure passed to the worker and never NULL. The NULL check on the
result of container_of() is therefore unnecessary and misleading.
Remove it.

This change was made automatically with the following Coccinelle script.

@@
type t;
identifier v;
statement s;
@@

<+...
(
  t v = container_of(...);
|
  v = container_of(...);
)
  ...
  when != v
- if (\( !v \| v == NULL \) ) s
...+>

Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>