drm/amdgpu: Enable HDP memory light sleep

Due to the register name and setting change of HDP
memory light sleep on Vega20,change accordingly in
the driver.

Signed-off-by: Kenneth Feng <kenneth.feng@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>