sisfb: limit POST memory test according to PCI resource length

If the POST memory test fails, the driver may access illegal
memory areas. Instead of hard coding the maximum size, set it
according to the PCI resource length (an additional check is needed in
sisfb_post_map_vram() to ensure it's big enough). DRAM sizing will later
adjust video_size to the correct value.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Thomas Winischhofer <thomas@winischhofer.net>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>