randr: Re-add removed NULL checks to xf86RandR12.c

Commit 1e3f9ea1 removed some NULL checks from xf86RandR12.c, on the premise that
they can't be reached unless RandR has already been initialized. For threesuch
calls, that's not true:

xf86Crtc.c::xf86CrtcScreenInit():

    if (c == config->num_crtc) {
        xf86RandR12SetRotations(screen, RR_Rotate_0 | RR_Rotate_90 |
                                RR_Rotate_180 | RR_Rotate_270 |
                                RR_Reflect_X | RR_Reflect_Y);
        xf86RandR12SetTransformSupport(screen, TRUE);
    }
    else {
        xf86RandR12SetRotations(screen, RR_Rotate_0);
        xf86RandR12SetTransformSupport(screen, FALSE);
    }

xf86Crtc.c::xf86CrtcCloseScreen():

    xf86RandR12CloseScreen(screen);

This change adds checks back to xf86RandR12Set{Rotations,TransformSupport}() and
xf86RandR12CloseScreen(), checking that xf86RandR12KeyRec has been registered.
Without this, X will hit an assert that causes it to abort.

Signed-off-by: Alex Goins <agoins@nvidia.com>
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