drm/ttm: decrease ttm bo priority number

decrease and also reserve priority number for KFD using

Signed-off-by: Roger.He <[e-mail]>
Reviewed-by: Christian König <[e-mail]>
Reviewed-by: Alex Deucher <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

include/drm/ttm/ttm_bo_driver.h

···
 #include <linux/spinlock.h>
 #include <linux/reservation.h>
 
-#define TTM_MAX_BO_PRIORITY	16U
+#define TTM_MAX_BO_PRIORITY	4U
 
 struct ttm_backend_func {
 	/**