drm/xe/hwmon: Add kernel doc and refactor xe hwmon

Add kernel doc and refactor some of the hwmon functions, there is
no functionality change.

Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Cc: Ashutosh Dixit <ashutosh.dixit@intel.com>
Signed-off-by: Badal Nilawar <badal.nilawar@intel.com>
Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231030115618.1382200-2-badal.nilawar@intel.comSigned-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>