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 <christian.koenig@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>