Allow for missing or disabled compat_output

When the compat output is missing (I don't think this is actually
possible), or is disabled (and hence has no crtc), we would like to
avoid dereferencing NULL pointers. This patch creates inline functions
to extract the current compat output, crtc or associated RandR crtc
structure, carefully checking for NULL pointers everywhere.

Signed-off-by: Keith Packard <keithp@keithp.com>