drm/xe: Build PM into GuC CT layer

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