vgacon: Fall back to the dummy console if mapping VGA memory fails

On Alpha, setup_arch() registers vgacon whenever CONFIG_VGA_CONSOLE is
enabled. On Marvel (EV7) systems whose SRM console is serial, no VGA
hose is set, so marvel_ioremap() finds no hose for 0xb8000 and returns
NULL. vgacon_startup() then probes video memory through that NULL
pointer and oopses in console_init():

  Unable to handle kernel paging request at virtual address 0000000000000000
  pc is at vgacon_startup+0x174/0x910

VGA_MAP_MEM() is ioremap() on Alpha and powerpc and can fail. Treat
failure like a failed probe and fall back to the dummy console. If a
VGA card is present, locate_and_init_vga() still switches to vgacon
once PCI is up.

Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Magnus Lindholm <linmag7@gmail.com>
Tested-by: Magnus Lindholm <linmag7@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>