drm/i915: Add the display switch logic for vGPU in i915 driver

Display switch logic is added to notify the host side that
current vGPU have a valid surface to show. It does so by
writing the display_ready field in PV INFO page, and then
will be handled in the host side. This is useful to avoid
trickiness when the VM's framebuffer is being accessed in
the middle of VM modesetting, e.g. compositing the framebuffer
in the host side.

v2:
        - move the notification code outside the 'else' in load sequence
        - remove the notification code in intel_crtc_set_config()

v4:
        - code rebase, no need to define another dev_priv
        - use #define instead of enum for display readiness

Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
Signed-off-by: Jike Song <jike.song@intel.com>
Signed-off-by: Zhiyuan Lv <zhiyuan.lv@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>