Take PM ref when any G2H are outstanding, drop when none are
outstanding.

To safely ensure we have PM ref when in the GuC CT layer, a PM ref needs
to be held when scheduler messages are pending too.

v2:
 - Add outer PM protections to xe_file_close (CI)
v3:
 - Only take PM ref 0->1 and drop on 1->0 (Matthew Auld)
v4:
 - Add assert to G2H increment function
v5:
 - Rebase
v6:
 - Declare xe as local variable in xe_file_close (CI)

Fixes: dd08ebf6 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240719172905.1527927-5-matthew.brost@intel.com