I've been attempting to solve a bug with i915 & switcheroo on many Macbook 
Pros (some with nvidia discrete cards, others radeon), report here:

https://bugs.freedesktop.org/show_bug.cgi?id=61115

Basically, after switching to i915 using switcheroo the display isn't 
connected. It was mentioned that some kind of reprobing was needed, so I tried 
the following:

diff --git a/drivers/gpu/drm/i915/i915_dma.c b/drivers/gpu/drm/i915/i915_dma.c
index d443441..fbc0d1c 100644
--- a/drivers/gpu/drm/i915/i915_dma.c
+++ b/drivers/gpu/drm/i915/i915_dma.c
@@ -1281,6 +1281,13 @@ static void i915_switcheroo_set_state(struct pci_dev 
*pdev, enum vga_switcheroo_
 	}
 }
 
+static void
+i915_switcheroo_reprobe(struct pci_dev *pdev)
+{
+	struct drm_device *dev = pci_get_drvdata(pdev);
+	intel_fbdev_output_poll_changed(dev);
+}
+
 static bool i915_switcheroo_can_switch(struct pci_dev *pdev)
 {
 	struct drm_device *dev = pci_get_drvdata(pdev);
@@ -1295,7 +1302,7 @@ static bool i915_switcheroo_can_switch(struct pci_dev 
*pdev)
 
 static const struct vga_switcheroo_client_ops i915_switcheroo_ops = {
 	.set_gpu_state = i915_switcheroo_set_state,
-	.reprobe = NULL,
+	.reprobe = i915_switcheroo_reprobe,
 	.can_switch = i915_switcheroo_can_switch,
 };
 
Unfortunately that hasn't solved it. I've attached my dmesg too, since I think 
the following line is related to the problem:

[Firmware Bug]: ACPI(GFX0) defines _DOD but not _DOS

I'd be very grateful if somebody with a better understanding of this could 
take a look - I'll happily test any patches.