intelfb: add vsync interrupt support

[03/05] intelfb: Implement basic interrupt handling

Functions have been added to enable and disable interrupts using the MMIO registers. Currently only pipe A vsync interrupts are enabled.
A generalized vsync accounting struct is defined, with the intent that it can encapsulate per-pipe vsync related info in the future. Currently a single instance is hard-coded.
The interrupt service routine currently only looks for vsync interrupts on pipe A, and increments a counter and wakes up anyone waiting on it.

This implementation is heavily influenced by similar implementations in the atyfb and matroxfb drivers.

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