drm/i915: make intel_gvt_init() later instead of too early

Previously intel_gvt_init() was called very early even before
MMIO initialization which had several drawbacks:
- Have to handle MMIO access for initial MMIO state dump if golden
  state firmware is not available
- Hypervisor detection should depend on pvinfo only instead of detecting
  hypervisor status.
- Don't know hw resource size e.g aperture, ggtt size to determine
  for vGPU type, etc.

This trys to move intel_gvt_init() call late after required info
has already been initialized for GVT host.

Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>