drm/amd/pm: don't update runpm last_usage on debugfs getter

drm/amd/pm: don't update runpm last_usage on debugfs getter

Reading pm values from the GPU shouldn't prevent it to be suspended
by resetting the last active timestamp (eg: if an background app
monitors GPU sensors every second, it would prevent the autosuspend
sequence to trigger).

Tested-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>