staging: xgifb: copy PCI ROM properly

Use proper helper functions to copy the PCI ROM. Also use dynamic memory
allocation. The original code mapped incorrect amount of memory and will
crash on some platforms.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>