drm/xe/display: Improve s2idle handling.

We accidentally always pass true as s2idle argument, instead of
calculating it in the same way as i915.

Suspend modes were removed to achieve compatibility with i915, but
accidentally left in the source code.

While at it, fix all other cases too, s2idle will go into a D1 state and
setting a lower power state should be handled by PCI core.

Maybe my laptop stops draining so much power during suspend now? I can
only hope..

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>