Add a common allocation helper. Cleaning up the mix of kzalloc/kmalloc
and some unused code in the selftest.

v2: polish kernel doc a bit
v3: polish kernel doc even a bit more

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210611120301.10595-3-christian.koenig@amd.com