Correct resolution in Xorg with Intel KMS and monitor enabled or disabled while booting the system

kms_no_monitor_config.txt

Instructions below - meant for versions prior to x11-base/xorg-drivers-1.19 - will work unless one hits this bug:
+ https://bugs.freedesktop.org/show_bug.cgi?id=92792

Additional three steps to success. The focus lies on "booting with monitor turned on or off".
Therefore I assume KMS works as expected and edid.bin exists inside /lib/firmware/edid.
Adjust your output connector and resolutions in all configs accordingly:

1. Kernel config (tested with 4.9.9)
+ CONFIG_FIRMWARE_IN_KERNEL=y
+ CONFIG_EXTRA_FIRMWARE="edid/edid.bin"
+ CONFIG_EXTRA_FIRMWARE_DIR="/lib/firmware"
+ CONFIG_DRM_LOAD_EDID_FIRMWARE=y

2. Bootloader options
+ drm_kms_helper.edid_firmware=HDMI-A-3:edid/edid.bin
+ video=HDMI-A-3:e # force enable connector - not always necessary

3. Minimal Xorg config
Section "Device"
    Identifier "Device0"
    Driver     "intel"
EndSection

Section "Monitor"
    Identifier "Monitor0"
    Option     "PreferredMode" "1920x1080"
EndSection

Section "Screen"
    Identifier   "Screen0"
    Device       "Device0"
    Monitor      "Monitor0"
    DefaultDepth 24

    SubSection "Display"
        Depth   24
	Modes   "1920x1080" "1280x720"
    EndSubSection
EndSection