Add i915 ioctls to configure pipes for vblank interrupt.

i915 vblanks can be generated from either pipe a or b, however a disabled
pipe generates no interrupts. This change allows the X server to select
which pipe generates vblank interrupts.

From: Keith Packard <keith.packard@intel.com> via DRM CVS
Signed-off-by: Dave Airlie <airlied@linux.ie>