drm/i915/gvt: Move mdev attribute groups into kvmgt module

As kvmgt module contains all handling for VFIO/mdev, leaving mdev attribute
groups in gvt module caused dependency issue. Although it was there for possible
other hypervisor usage, that turns out never to be true. So this moves all mdev
handling into kvmgt module completely to resolve dependency issue.

With this fix, no config workaround is required. So revert previous workaround
commits: adaeb718 ("vfio/gvt: fix DRM_I915_GVT dependency on VFIO_MDEV")
and 07e543f4 ("vfio/gvt: Make DRM_I915_GVT depend on VFIO_MDEV").

Reviewed-by: Colin Xu <colin.xu@intel.com>
Cc: Arnd Bergmann <arnd@kernel.org>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20210513083902.2822350-1-zhenyuw@linux.intel.com