vga_switcheroo: Use enum vga_switcheroo_state instead of int

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>