drm/amd/pm: fulfill the missing enablement for vega12/vega20 L2H and H2L interrupts

The feature mask bit was not correctly cleared. Without that, the L2H
and H2L interrupts cannot be enabled.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>