drm: Add start-up delay until a graphics device is present

The initialization of a graphics driver could be slower than Weston, leading Weston to fail.

So, a wait and probe loop is added to wait until any graphics be present, and then, proceed with DRM-backend initialization. If no graphics card is available in the timeout frame window, Weston will fail as expected.

That is made by adding a loop that sleeps the process for 100ms and probes the udev/DRM. Until either, a card to be found or 15 loops succeed(1.5s timeout).

Closes: #173
Signed-off-by: Igor Matheus Andrade Torrente igortorrente@outlook.com