drm/panthor: Don't return NULL from panthor_vm_get_heap_pool()

The kernel doc says this function returns either a valid pointer
or an ERR_PTR(), but in practice this function can return NULL if
create=false. Fix the function to match the doc (return
ERR_PTR(-ENOENT) instead of NULL) and adjust all call-sites
accordingly.

Fixes: 4bdca115 ("drm/panthor: Add the driver frontend block")
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240402141412.1707949-1-harshit.m.mogalapalli@oracle.com