drm/i915/gvt: move mmio init/clean function to mmio.c

Move the mmio space inititation function setup_vgpu_mmio()
and cleanup function clean_vgpu_mmio() in vgpu.c to dedicated
source file mmio.c, and rename them as intel_vgpu_init_mmio()
and intel_vgpu_clean_mmio() respectively.

Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>