staging: xgifb: setup initial video RAM size for systems without BIOS

On embedded systems without a BIOS, the DRAM sizing register is 0 after
the boot, and the driver thinks we have only 1 MB video memory. On such
cases, use the PCI window size for the initial size. This information
is needed only to create the I/O mapping - the driver will later detect
and configure the memory size correctly. Limit the size to 16 MB as it
should be sufficient for all supported video modes.

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