intelfb: add vsync interrupt support

[02/05] intelfb: Add interrupt related register definitions

Add constants for accessing HWSTAM, IER, IIR, and IMR registers.
Add constants for interrupt types supported by the 8xx and 9xx chipsets.
The registers are also stored in the hwstate struct and dumped in the debug routine.

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