drm/i915/gvt: Rename file render.{c, h} to mmio_context.{c, h}

Rename the files to reflect their real role - to switch the mmio context of
each vGPU engine.

v2: update Makefile.

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