drm/amdgpu/atom: Check kcalloc() for WS buffer in amdgpu_atom_execute_table_locked()

kcalloc() may fail. When WS is non-zero and allocation fails, ectx.ws
remains NULL while ectx.ws_size is set, leading to a potential NULL
pointer dereference in atom_get_src_int() when accessing WS entries.

Return -ENOMEM on allocation failure to avoid the NULL dereference.

Signed-off-by: Guangshuo Li <lgs201920130244@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>