compositor-drm: allocate CRTC on enable()

A drm_output needs a CRTC only when it is in use. Allocating a CRTC on
creation of drm_output will reserve the CRTC regardless of whether the
output is actually used or not. This may cause creating other
drm_outputs to fail if there are not enough CRTCs.

Instead, allocate the CRTC on drm_output enable() time. A drm_output
will have a valid CRTC only while it is enabled.

This allows us to create drm_output objects arbitrarily and without a
head assignment, which is required by the head-based output API for the
backends. The assigned heads will be known only at enable() time.

Now drm_output_enable() has to call drmModeGetResources() to be able to
find a suitable CRTC. We might want to cache the resources somewhere,
but that is it topic for another patch.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>