drm/amdkfd: fix a dereference of pdd before it is null checked

Currently pointer pdd is being dereferenced when assigning pointer
dpm and then pdd is being null checked.  Fix this by checking if
pdd is null before the dereference of pdd occurs.

Addresses-Coverity: ("Dereference before null check")
Fixes: 32cb59f3 ("drm/amdkfd: Track SDMA utilization per process")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>