drm/xe: Hold a PM ref when GT TLB invalidations are inflight

Avoid GT TLB invalidation timeouts by holding a PM ref when
invalidations are inflight.

v2:
 - Drop PM ref before signaling fence (CI)
v3:
 - Move invalidation_fence_signal helper in tlb timeout to previous
   patch (Matthew Auld)

Fixes: dd08ebf6 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
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: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240719172905.1527927-4-matthew.brost@intel.com