drm/i915: Extract intel_fifo_underrun.c

Prep work for some nice documentation. Requires that we export the
display irq enable/disable functions on ilk/ibx. But we already export
them for vlv/i915. So not more inconsistency.

v2: Rebase on top of skl stage 1.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
diff --git a/drivers/gpu/drm/i915/Makefile b/drivers/gpu/drm/i915/Makefile
index 3a6bce0..75fd7de 100644
--- a/drivers/gpu/drm/i915/Makefile
+++ b/drivers/gpu/drm/i915/Makefile
@@ -45,6 +45,7 @@
 # modesetting core code
 i915-y += intel_bios.o \
 	  intel_display.o \
+	  intel_fifo_underrun.o \
 	  intel_frontbuffer.o \
 	  intel_modes.o \
 	  intel_overlay.o \