drm/xe: Prepare display for D3Cold

Prepare power-well and DC handling for a full power
lost during D3Cold, then sanitize it upon D3->D0.
Otherwise we get a bunch of state mismatch.

Ideally we could leave DC9 enabled and wouldn't need
to move DC9->DC0 on every runtime resume, however,
the disable_DC is part of the power-well checks and
intrinsic to the dc_off power well. In the future that
can be detangled so we can have even bigger power savings.
But for now, let's focus on getting a D3Cold, which saves
much more power by itself.

v2: create new functions to avoid full-suspend-resume path,
which would result in a deadlock between xe_gem_fault and the
modeset-ioctl.

v3: Only avoid the full modeset to avoid the race, for a more
robust suspend-resume.

Cc: Anshuman Gupta <[email protected]>
Cc: Uma Shankar <[email protected]>
Tested-by: Francois Dugast <[email protected]>
Reviewed-by: Anshuman Gupta <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Signed-off-by: Rodrigo Vivi <[email protected]>

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