drm/i915: enable to read CSB and CSB write pointer from HWSP in GVT-g VM

Let GVT-g VM read the CSB and CSB write pointer from virtual HWSP, not all
the host support this feature, need to check the BIT(3) of caps in PVINFO.

v3 : Remove unnecessary comments.
v4 : Separate VM enable patch with GVT-g implementation patch due to code
dependency.
v5 : Use inline for GVT virtual HWSP caps check function.
v6 : Comments refine.

Signed-off-by: Weinan Li <weinan.z.li@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1508039725-1066-1-git-send-email-weinan.z.li@intel.com

drivers/gpu/drm/i915/i915_vgpu.h

@@ -30,6 +30,12 @@ void i915_check_vgpu(struct drm_i915_private *dev_priv);
 
 bool intel_vgpu_has_full_48bit_ppgtt(struct drm_i915_private *dev_priv);
 
+static inline bool
+intel_vgpu_has_hwsp_emulation(struct drm_i915_private *dev_priv)
+{
+	return dev_priv->vgpu.caps & VGT_CAPS_HWSP_EMULATION;
+}
+
 int intel_vgt_balloon(struct drm_i915_private *dev_priv);
 void intel_vgt_deballoon(struct drm_i915_private *dev_priv);