Introduce a PV INFO structure, to facilitate the Intel GVT-g
technology, which is a GPU virtualization solution with mediated
pass-through. This page contains the shared information between
i915 driver and the host emulator. For now, this structure utilizes
an area of 4K bytes on HSW GPU's unused MMIO space. Future hardware
will have the reserved window architecturally defined, and layout
of the page will be added in future BSpec.

The i915 driver load routine detects if it is running in a VM by
reading the contents of this PV INFO page. Thereafter a flag,
vgpu.active is set, and intel_vgpu_active() is used by checking
this flag to conclude if GPU is virtualized with Intel GVT-g. By
now, intel_vgpu_active() will return true, only when the driver
is running as a guest in the Intel GVT-g enhanced environment on
HSW platform.

v2:
take Chris' comments:
        - call the i915_check_vgpu() in intel_uncore_init()
        - sanitize i915_check_vgpu() by adding BUILD_BUG_ON() and debug info
take Daniel's comments:
        - put the definition of PV INFO into a new header - i915_vgt_if.h
other changes:
        - access mmio regs by readq/readw in i915_check_vgpu()

v3:
take Daniel's comments:
        - move the i915/vgt interfaces into a new i915_vgpu.c
        - update makefile
        - add kerneldoc to functions which are non-static
        - add a DOC: section describing some of the high-level design
        - update drm docbook
other changes:
        - rename i915_vgt_if.h to i915_vgpu.h

v4:
take Tvrtko's comments:
        - fix a typo in commit message
        - add debug message when vgt version mismatches
        - rename low_gmadr/high_gmadr to mappable/non-mappable in PV INFO
          structure
Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
Signed-off-by: Jike Song <jike.song@intel.com>
Signed-off-by: Eddie Dong <eddie.dong@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>