drm/radeon: fix underflow in r600_cp_dispatch_texture()

The "if (pass_size > buf->total)" can underflow so I have changed the
type of size and pass_size to unsigned to avoid this problem.

Reviewed-by: Christian König <[e-mail]>
Signed-off-by: Dan Carpenter <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/radeon/r600_cp.c

···
 	struct drm_buf *buf;
 	u32 *buffer;
 	const u8 __user *data;
-	int size, pass_size;
+	unsigned int size, pass_size;
 	u64 src_offset, dst_offset;
 
 	if (!radeon_check_offset(dev_priv, tex->offset)) {