intelfb: add vsync interrupt support

[05/05] intelfb: Honor FB_ACTIVATE_VBL for display panning

Extends the intelfb_vsync struct to store panning offset. The interrupt service routine uses the stored panning offset if a pan is requested for the vsync. intelfbhw_disable_irq also pans the display if there is a pending request.

Signed-off-by: Eric Hustvedt <ehustvedt@cecropia.com>