drm/amd/pm: invalidate hdp before CPU access the memory written by GPU

To eliminate the possible influence by outdated HDP read cache.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>