drm/vmwgfx: Rename dummy to is_iomem

Rename dummy to is_iomem because that's what it is even if we're not
activelly using it. Makes the code easier to read.

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/20230131033542.953249-7-zack@kde.org