modesetting: Fix ms_covering_crtc() segfault with non-xf86Crtc slave

DIX ScreenRecs don't necessarily have an xf86CrtcConfigPtr DDX private.
ms_covering_crtc() assumes that they do, which can result in a segfault.

Update ms_covering_crtc() to check the XF86_CRTC_CONFIG_PTR() returned pointer
before dereferencing it. This will still mean that ms_covering_crtc() can't fall
back to the primary output when a drawable overlaps a slave output (going to the
1 FPS default instead), but it won't segfault.

Signed-off-by: Alex Goins <agoins@nvidia.com>
(cherry picked from commit 797e7a0c)