x86, setup: When restoring the screen, update boot_params.screen_info

When we restore the screen content after a mode change, we return the
cursor to its former position.  However, we need to also update
boot_params.screen_info accordingly, so that the decompression code
knows where on the screen the cursor is.  Just in case the video BIOS
does something extra screwy, read the cursor position back from the
BIOS instead of relying on it doing the right thing.

While we're at it, make sure we cap the cursor position to the new
screen coordinates.

Reported-by: Wim Osterholt <wim@djo.tudelft.nl>
Bugzilla-Reference: http://bugzilla.kernel.org/show_bug.cgi?id=15329
Signed-off-by: H. Peter Anvin <hpa@zytor.com>