drm/vmwgfx: Use the common gem mmap instead of the custom code

Before vmwgfx supported gem it needed to implement the entire mmap logic
explicitly. With GEM support that's not needed and the generic code
can be used by simply setting the vm_ops to vmwgfx specific ones on the
gem object itself.

Removes a lot of code from vmwgfx without any functional difference.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Reviewed-by: Maaz Mombasawala <mombasawalam@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230131033542.953249-2-zack@kde.org