drm/i915/gvt: vGPU interrupt virtualization.

This patch introduces vGPU interrupt emulation framework.

The vGPU intrerrupt emulation framework is an event-based interrupt
emulation framework. It's responsible for emulating GEN hardware interrupts
during emulating other HW behaviour.

It consists several components:

- Descriptions of interrupt register bit
- Upper level <-> lower level interrupt mapping
- GEN HW IER/IMR/IIR register emulation routines
- Event-based interrupt propagation interface

When a GVT-g component wants to inject an interrupt to a VM during a
emulation, first it should specify the event needs to be emulated and the
framework will deal with the rest of emulation:

- Generating related virtual IIR bit according to virtual IER and IMRs,
- Generate related virtual upper level virtual IIR bit accodring to the
per-platform interrupt mapping
- Injecting a MSI to VM

Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>