libdrm: speed up connector & mode fetching

This patch speeds up drmModeGetConnector by pre-allocating mode &
property info space before calling into the kernel.  In many cases this
pre-allocation will be sufficient to hold the returned values (it's easy
enough to tweak if the common case becomes larger), which means we don't
have to make the second call, which saves a lot of time.

Acked-by: Jakob Bornecrantz <wallbraker@gmail.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>