drm/i915: Support alias ppgtt in VM if ppgtt is enabled

The current Intel GVT-g only supports alias ppgtt. And the
emulation is done in the host by first trapping PP_DIR_BASE
mmio accesses. Updating PP_DIR_BASE by using instructions such
as MI_LOAD_REGISTER_IMM are hard to detect and are not supported
in current code. Therefore this patch also adds a new callback
routine - vgpu_mm_switch() to set the PP_DIR_BASE by mmio writes.

v2:
take Chris' comments:
        - move the code into sanitize_enable_ppgtt()
v4:
take Tvrtko's comments:
        - fix the parenthesis alignment warning

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