drm/xe: Release runtime pm for error path of xe_devcoredump_read()

drm/xe: Release runtime pm for error path of xe_devcoredump_read()

xe_pm_runtime_put() is missed to be called for the error path in
xe_devcoredump_read().
Add function description comments for xe_devcoredump_read() to help
understand it.

v2: more detail function comments and refine goto logic (Matt)

Fixes: c4a2e5f865b7 ("drm/xe: Add devcoredump chunking")
Cc: stable@vger.kernel.org
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Shuicheng Lin <shuicheng.lin@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250707004911.3502904-6-shuicheng.lin@intel.com