Ubuntu 13.10 installation on iMac 21,5 inch radeon 4670 black screen

Dear Ubuntu community,

I have installed Ubuntu 13.10 on iMac 21,5 inch mid 2010 with radeon 4670...
I am Linux user for more than 12 years...but this is first time i am willing to install it on my iMac and change osx for Linux...
It is working realy great...BUT...

To be able to install it i needed to connect extern monitor on my displayport and that way my iMac monitor would became active...Ones active i can disconect hdmi cable (displayport) and remains active...
When i restart iMac (with extern monitor disconected) i see grub loader and i can choose ¨Ubuntu¨...it starts up , but iMac monitor stays inactive...than after couple seconds i hear start up chime...it comes to log in without problem but i can´t see it.... Than i connect extern monitor (TV) and intern iMac monitor became active again...

When is active everything is working great...i have 3d acceleration, i use steam on it...works great, no problem...

I have tried radeon.modeset=0 and nomodeset and single 1 nomodeset.... i tried even to play with xrandr -q , xrandr --output eDP --mode 1920x1080... no results... i must connect extern monitor to it to became active...it is very anoying

I hope you guys have solution for me...i really wanna stay on ubuntu...

Thank you in advance

Best regards

Miki