drm/gpuvm: Dual-licence the drm_gpuvm code GPL-2.0 OR MIT

Dual-licence in order to make it possible for other non-GPL os'es
to re-implement the code. The use of EXPORT_SYMBOL_GPL() is intentionally
left untouched to prevent use of drm_gpuvm as a proxy for non-GPL drivers
to access GPL-only kernel symbols.

Much of the ideas and algorithms used in the drm_gpuvm code is already
present in one way or another in MIT-licensed code.

Cc: Danilo Krummrich <dakr@redhat.com>
Cc: airlied@gmail.com
Cc: daniel@ffwll.ch
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Danilo Krummrich <dakr@redhat.com>
Reviewed-by: Francois Dugast <francois.dugast@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231010142725.8920-1-thomas.hellstrom@linux.intel.com