drm/i915: Disable framebuffer compression for i915 driver in VM

Framebuffer compression is disabled when driver detects it's
running in a Intel GVT-g enlightened VM, because FBC is not
emulated and there is no stolen memory for a vGPU.

v2:
take Chris' comments:
        - move the code into intel_update_fbc()

v4:
take Tvrtko's comments:
        - rebase the code into intel_fbc_update()

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>