Push the test for info->screen_base from fb_read() and fb_write() into
the implementations of struct fb_ops.{fb_read,fb_write}. In cases where
the driver operates on info->screen_buffer, test this field instead.

While bothi fields, screen_base and screen_buffer, are stored in the
same location, they refer to different address spaces. For correctness,
we want to test each field in exactly the code that uses it.

v2:
	* also test screen_base in pvr2fb (Geert)
	* also test screen_buffer in ivtvfb, arcfb, broadsheetfb,
	  hecubafb, metronomefb and ssd1307fb (Geert)
	* give a rational for the change (Geert)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Sui Jingfeng <suijingfeng@loongson.cn>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230428122452.4856-18-tzimmermann@suse.de