configure.ac: Allow forcible disabling of Cairo support

We don't want to build libdrm tests with Cairo support under Poky, since
they're never used and also cause a build loop from libdrm -> cairo ->
mesa-dri -> libdrm.

To avoid variance in build results, introduce a --disable-cairo-tests
switch.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>