If your kernel has i915 driver enabled, the following (which I quote from ArchLinux Wiki, and which in turn quotes from nouveau wiki) might prove helpful.
Basically you could experiment with adding
video=... parameter in grub using the
d option (see the parameter format below) to disable the internal monitor (whichever name it might have for
<conn> option you need to find from the system log which should display the available options during kernel start-up. The information might still be available from previous start-up in
/var/log/dmesg if your distro has that, please try
grep i915 /var/log/dmesg* and post it here.
A mode can be forced on the kernel command line. Unfortunately, the
command line option video is poorly documented in the DRM case. Bit
and pieces on how to use it can be found in
The format is:
<conn>: Connector, e.g. DVI-I-1, see your kernel log.
<xres> x <yres>: resolution
M: compute a CVT mode?
R: reduced blanking?
-<bpp>: color depth
@<refresh>: refresh rate
i: interlaced (non-CVT mode)
e: output forced to on
d: output forced to off
D: digital output forced to on (e.g. DVI-I connector)
You can override the modes of several outputs using "video" several
times, for instance, to force DVI to 1024x768 at 85 Hz and TV-out off: