drm/xe/hwmon: Use devm_mutex_init()

Use devm_mutex_init() instead of hand-writing it.

This saves some LoC, improves readability and saves some space in the
generated .o file.

Before:
======
   text	   data	    bss	    dec	    hex	filename
  36884	  10296	     64	  47244	   b88c	drivers/gpu/drm/xe/xe_hwmon.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  36651	  10224	     64	  46939	   b75b	drivers/gpu/drm/xe/xe_hwmon.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Stuart Summers <stuart.summers@intel.com>
Link: https://lore.kernel.org/r/989e96369e9e1f8a44b816962917ec76877c912d.1757252520.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>