drm/xe: Document Xe PM component

Replace outdated information with a proper PM documentation.
Already establish the rules for the runtime PM get and put that
Xe needs to follow.

Also add missing function documentation to all the "exported" functions.

v2: updated after Francois' feedback.
    s/grater/greater (Matt)
v3: detach D3 from runtime_pm
    remove opportunistic S0iX (Anshuman)

Cc: Matthew Auld <[email protected]>
Cc: Anshuman Gupta <[email protected]>
Acked-by: Francois Dugast <[email protected]>
Reviewed-by: Francois Dugast <[email protected]> #v2
Signed-off-by: Rodrigo Vivi <[email protected]>
Reviewed-by: Anshuman Gupta <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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