Don't recall exactly, since Jessie probably my dCore system's have required setting 'Device" option:
Option "DRI" "false"
Running dCore-Stretch, for example, if DRI option not set to false the system boots to black screen, no mouse pointer, can't exit without REISUB. With the DRI false option can't run glxgears, reported swrast issue
On this old hardware, i believe i'm just getting software rendering at this point. Nonetheless loading the 'intel' driver works well, xorg.conf enables the desired resolution and DRI false enables boot with Xorg errors.
I've noticed issues between xorg-all and xorg-<specific> before but couldn't pinpoint. Looking at the extensions, it's apparent xorg-all includes more than just additional graphic hardware drivers.
Here's a mesa comparison from dCore-Jessie, there are probably other additional packages too:
Using xorg-intel in dCore-Stretch, when libgl1-mesa-dri is loaded then the xorg.confg DRI false option is no longer required.
Similarly, in dCore-Jessie loading libgl1-mesa-swx11 fixes the issue, enables glxgears (~60fps), less Xorg log reports.
Not sure if extra 'mesa' packages should be added as dependencies to all xorg-<specific> drivers? It would probably prevent black screen boots for some hardware when switching from xorg-all to an xorg-<specific> driver, but the extensions would be larger.