drm/xe/hwmon: Update xe_hwmon_get_reg to return struct xe_reg

Return struct xe_reg instead of reg.raw from xe_hwmon_get_reg
to have abstracted usage of struct xe_reg.

v2:
 - Use xe_reg_is_valid function instead of XE_REG_IS_VALID macro
   as it is removed.

Signed-off-by: Karthik Poosa <karthik.poosa@intel.com>
Suggested-by: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Badal Nilawar <badal.nilawar@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240405130127.1392426-3-karthik.poosa@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>