drm/xe: Make local functions static

A few static functions not being declared like that break the build with
W=1, like e.g.

	cc1: all warnings being treated as errors
	make[2]: *** [../scripts/Makefile.build:250: drivers/gpu/drm/xe/xe_gt.o] Error 1
	../drivers/gpu/drm/xe/xe_guc.c:240:6: error: no previous prototype for ‘guc_write_params’ [-Werror=missing-prototypes]
	  240 | void guc_write_params(struct xe_guc *guc)
	      |      ^~~~~~~~~~~~~~~~

Make them static.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>