drm/i915/gtt: Revert "Disable read-only support under GVT"

This reverts commit c9e66688.

Checked GVT codes that guest PPGTT PTE flag bits are propagated
to shadow PTE. Read/write bit is not changed. Further tested by
i915 self-test case "igt_ctx_readonly". No error or GPU hang was
detected. So enable read-only support under GVT.
Signed-off-by: Hang Yuan <hang.yuan@linux.intel.com>
Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1540883281-11359-1-git-send-email-hang.yuan@linux.intel.comAcked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -1625,12 +1625,8 @@ static struct i915_hw_ppgtt *gen8_ppgtt_create(struct drm_i915_private *i915)
 		1ULL << 48 :
 		1ULL << 32;
 
-	/*
-	 * From bdw, there is support for read-only pages in the PPGTT.
-	 *
-	 * XXX GVT is not honouring the lack of RW in the PTE bits.
-	 */
-	ppgtt->vm.has_read_only = !intel_vgpu_active(i915);
+	/* From bdw, there is support for read-only pages in the PPGTT. */
+	ppgtt->vm.has_read_only = true;
 
 	i915_address_space_init(&ppgtt->vm, i915);