Comment 17 for bug 899244

The name is very poor, but the poulsbo driver is not a "video driver" , it is a driver for the backlight

From the help:

Symbol: STUB_POULSBO [=n] │
  │ Type : tristate │
  │ Prompt: Intel GMA500 Stub Driver │
  │ Defined at drivers/gpu/stub/Kconfig:1 │
  │ Depends on: HAS_IOMEM [=y] && PCI [=y] && NET [=y] │
  │ Location: │
  │ -> Device Drivers │
  │ -> Graphics support │
  │ Selects: BACKLIGHT_CLASS_DEVICE [=y] && VIDEO_OUTPUT_CONTROL [=m] && INPUT [=y] && ACPI

So if the psb_gfx were to fail, it falls back to VESA , or fails completely (depends on the kernel), but it does not "fall back" to the poulsbo driver.

The backlight works WITHOUT the poulsbo driver.

well "works" with a script to manage brightness as my function keys do not always work, even with the kernel options "acpi_backlight=vendor and acpi_osi=Linux". Honestly I am not sure what the poulsbo driver is used for, AFIK it can be completely removed.