vgaarb: Add support for 64-bit frame buffer address

EFI GOP uses 64-bit frame buffer address when some BIOS
disabled CSM support. vgaarb only stores lfb_base,
this will lead boot framebuffer to wrong device.

Add ext_lfb_base support to use 64-bit fb address.

Signed-off-by: Aaron Ma <aaron.ma@canonical.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1535739600-8842-1-git-send-email-aaron.ma@canonical.com