Hi everyone. I've solved this problem by disabling BB-BONELT-HDMI, BB-BONELT-HDMIN, BB-BONE-EMMC-2G firmwares and by enabling BB-BONE-DVI firmware in my uEnv.txt:
optargs=capemgr.disable_partno=BB-BONELT-HDMI,BB-BONELT-HDMIN,BB-BONE-EMMC-2G capemgr.enable_partno=BB-BONE-DVI video=DVI-D-1:1024x600M@60