i965/iris: properly check if MADVISE_WILLNEED succeeds

When reusing a buffer, i965 and iris calls madvise ioctl with flag MADVISE_WILLNEED and retained sets to 1, then checks if it's changed or not. On rare occations, i915 might fail the call without setting retained to 0, the 3d driver reuses a buffer without backing storage in a batchbuffer and is rejected by i915 with errno EFAULT(bad address).

Signed-off-by: James Xiong james.xiong@intel.com