intel: Dump out memory usage information when the kernel fails to pin

The execbuffer ioctl returns ENOMEM when it fails to pin all of the buffers
in the GTT. This is usually caused by the DRM client attempting to use too
much memory in a single request. Dumping out the requested and available
memory values should help point out failures in the DRM code to catch over
commitments of this form.

Signed-off-by: Keith Packard <keithp@keithp.com>