drm/vmwgfx: Implement create_handle on drm_framebuffer_funcs

The fb handle code assumes it deals with GEM objects. Because vmwgfx
buffer objects were not actually GEM objects we were not able to
implement that interface. Now that vmwgfx supports GEM buffer objects
we can trivially implement create_handle for buffer object backed
framebuffers.
Among others this gets IGT's kms_getfb test passing.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211206172620.3139754-6-zack@kde.org