[Intel-gfx] refactor the i915 GVT support and move to the modern mdev API v2

[Christoph Hellwig]

On Thu, Nov 04, 2021 at 02:59:18PM +0200, Joonas Lahtinen wrote:
> The minimal we should do is to eliminate the double underscore
> prefixed functions. But I would prefer to have the symbol exports by
> default so that we can enable the functionality just by loading the
> module.

I'm fine with exporting by default, but just loading won't really work
even with that:

 - there are a bunch of IS_ENABLED conditionals in the i915 (although
   they look like minor optimizations to me).
 - the enable_gvt needs to be set, although after this refactor this
   option is completely pointless and should probably be enabled
 - the enable_guc option needs to be disable for gvt to work.