Modify xorg.conf_intel to disable DRI3/Present and use DRI2 instead.

Finally found a intel-ddx option that allows to disable DRI3/Present
and revert to good old proven DRI2, without cumbersome the need to
use setenv('LIBGL_DRI3_DISABLE','1') from within Octave/Matlab.

Adding these disable options is a much more pain-free workaround for
us until we find a proper fix for the upstream server/ddx.

Disabling DRI3 via the "DRI" "2" option is neccessary, as just
disabling "Present" doesn't help. Disabling "Present" in addition
is not really needed. Maybe we deal with a DRI3 bug, or maybe it
is a Present bug and use of DRI3 without Present simply doesn't
work?

-> xorg.conf_intel updated.
-> Successfully tested on KUbuntu 14.10, XOrg 1.16.0, Mesa 10.3.0.