drm/amdkfd: Reuse function to find a process through pasid

The kfd_lookup_process_by_pasid() is just for that purpose,
so use it instead of repeating the code.

v2: return on the condition (p == NULL) instead of BUG_ON(!p).

Signed-off-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>