vesa: flush lfb after zeroing

vesa: flush lfb after zeroing

If Xen is going to relinquish the VGA console, flush the linear frame
buffer after zeroing it in vesa_endboot().

Failing to do so in some circumstances leads to the actual linear
framebuffer on the graphics card still containing the output of the
Xen boot console can lead to ugly graphics output when dom0 is setting
up the graphics card for its own use.

While the patch is quite large, it is mostly just code motion to
prevent having to forward declare lfb_flush().  The only functional
change to vesa_endboot() is to insert a call to lbf_flush().

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Committed-by: Keir Fraser <keir@xen.org>