drm: i810_dma.c: fix pointer arithmetic for 64-bit target

First warning result from open-coded PTR_ERR,
the rest is caused by code like this:

*(u32 *) ((u32) buf_priv->kernel_virtual + used)

I've also fixed a missing PTR_ERR in i830_dma.c

From: Denis Vlasenko <vda.linux@googlemail.com>
Signed-off-by: Dave Airlie <airlied@linux.ie>