staging: xgifb: vb_init: use readl()/writel() to access iomapped memory

Use readl() and writel() in FB memory test instead of direct pointer
access, and also add iomem annotations for the FB memory.

The patch eliminates the following sparse warnings:

drivers/staging/xgifb/XGI_main_26.c:2113:69: warning: incorrect type in assignment (different address spaces)
drivers/staging/xgifb/XGI_main_26.c:2113:69:    expected unsigned char *static [addressable] [toplevel] [assigned] pjVideoMemoryAddress
drivers/staging/xgifb/XGI_main_26.c:2113:69:    got void [noderef] <asn:2>*
drivers/staging/xgifb/XGI_main_26.c:2399:30: warning: incorrect type in assignment (different address spaces)
drivers/staging/xgifb/XGI_main_26.c:2399:30:    expected char [noderef] <asn:2>*screen_base
drivers/staging/xgifb/XGI_main_26.c:2399:30:    got char *[addressable] [toplevel] [assigned] video_vbase
drivers/staging/xgifb/XGI_main_26.c:2430:31: warning: incorrect type in argument 1 (different address spaces)
drivers/staging/xgifb/XGI_main_26.c:2430:31:    expected void volatile [noderef] <asn:2>*addr
drivers/staging/xgifb/XGI_main_26.c:2430:31:    got char *[addressable] [toplevel] [assigned] video_vbase
drivers/staging/xgifb/XGI_main_26.c:2454:31: warning: incorrect type in argument 1 (different address spaces)
drivers/staging/xgifb/XGI_main_26.c:2454:31:    expected void volatile [noderef] <asn:2>*addr
drivers/staging/xgifb/XGI_main_26.c:2454:31:    got char *[addressable] [toplev

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