drm/radeon/ni_dpm: remove redundant NULL check

'leakage_table' will always be successfully initialized as a pointer
to '&rdev->pm.dpm.dyn_state.cac_leakage_table'.

Remove unnecessary check if only to silence static checkers.

Found by Linux Verification Center (linuxtesting.org) with static
analysis tool Svace.

Fixes: 69e0b57a

 ("drm/radeon/kms: add dpm support for cayman (v5)")
Signed-off-by: Nikita Zhandarovich <n.zhandarovich@fintech.ru>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>