drm/xe: Document Xe PM component

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Replace outdated information with a proper PM documentation.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 <matthew.auld@intel.com>
Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Acked-by: Francois Dugast <francois.dugast@intel.com>
Reviewed-by: Francois Dugast <francois.dugast@intel.com> #v2
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240222163937.138342-1-rodrigo.vivi@intel.com