drm/i915: Add ppgtt to GVT GEM context

Currently the guest couldn't boot up under GVT-g environment as the
following call trace exists:
[  272.504762] BUG: unable to handle kernel NULL pointer dereference at 0000000000000100
[  272.504834] Call Trace:
[  272.504852]  execlists_context_pin+0x2b2/0x520 [i915]
[  272.504869]  intel_gvt_scan_and_shadow_workload+0x50/0x4d0 [i915]
[  272.504887]  intel_vgpu_create_workload+0x3e2/0x570 [i915]
[  272.504901]  intel_vgpu_submit_execlist+0xc0/0x2a0 [i915]
[  272.504916]  elsp_mmio_write+0xc7/0x130 [i915]
[  272.504930]  intel_vgpu_mmio_reg_rw+0x24a/0x4c0 [i915]
[  272.504944]  intel_vgpu_emulate_mmio_write+0xac/0x240 [i915]
[  272.504947]  intel_vgpu_rw+0x22d/0x270 [kvmgt]
[  272.504949]  intel_vgpu_write+0x164/0x1f0 [kvmgt]

GVT GEM context is created by i915_gem_context_create_gvt() which
doesn't allocate ppgtt. So GVT GEM context structure doesn't have
a valid i915_hw_ppgtt.

This patch create ppgtt table at GVT GEM context creation, then assign
shadow ppgtt's root table address to this ppgtt when shadow ppgtt will
be used on GPU. So GVT GEM context has valid ppgtt address. But note
that this ppgtt only contain valid ppgtt root table address, the table
entry in this ppgtt structure are invalid.

Fixes:4a3d3f67

("drm/i915: Match code to comment and enforce ppgtt for execlists")

Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/1539841231-3157-1-git-send-email-xiong.y.zhang@intel.com