modetest: introduce and use get_crtc_by_id() and get_crtc_mask()

Let's make the code shorter, this avoid crashes (when drmModeGetCrtc()
fails) by using a couple of helpers. As get_resources() considers the
drmModeGetCrtc() fail non-fatal, we might as well handle it properly.

v2: Add a comment above the unreachable abort() (Eze)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Ezequiel Garcia <ezequiel@collabora.com>
Tested-by: Ezequiel Garcia <ezequiel@collabora.com>