If you use vga=791, that mean you will use vesa driver to set mode. 1366x768 is
not a standard mode and normally will not be pre-set in the video bios. So you
can not find it and use VBE standard call to set/get this mode.
Maybe you can try `modprobe intelfb mode=1366x768...@50`.
[1] intelfb.tx
It is reported as https://bugs.freedesktop.org/show_bug.cgi?id=29716.
find the regression and here is the fix, maybe ugly.
diff --git a/src/intel_display.c b/src/intel_display.c
index d32224e..4a159bf 100644
--- a/src/intel_display.c
+++ b/src/intel_display.c
@@ -339,6 +339,8 @@ intel_crtc_apply