vgabios: Return from handle_1011() if handler found.

Fix cut-and-paste error in handle_1011() causing spurious debug_stub()
calls when CONFIG_VGA_STDVGA_PORTS is enabled.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
Update EFI_COMPATIBILITY16_TABLE to match 0.98 spec update

Unless CONFIG_MALLOC_UPPERMEMORY is turned off, we expect to use the
space between the top of option ROMs and the bottom of our own BIOS code
as a stack. OVMF was previously marking the whole region from 0xC0000 to
0xFFFFF read-only before invoking our Legacy16Boot method. Read-only
stack considered harmful.

Version 0.98 of the CSM spec adds the UmaAddress and UmaSize fields which
allow the CSM to specify a memory region that needs to be writeable, so
provide that information.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>