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

ms_covering_crtc() uses RRFirstOutput() to determine a primary output to fall
back to if a drawable is overlapping a slave output.

If the primary output is a slave output, RRFirstOutput() will return a slave
output even if passed a master ScreenPtr. ms_covering_crtc() dereferences the
output's devPrivate, which is invalid for non-modesetting outputs, and can
crash.

Changing RRFirstOutput() could have unintended side effects for other callers,
so this change replaces the call to RRFirstOutput() with ms_first_output().
ms_first_output() ignores the primary output if it doesn't match the given
ScreenPtr, choosing the first connected output instead.

Signed-off-by: Alex Goins <agoins@nvidia.com>
(cherry picked from commit 3ef9029ace)

hw/xfree86/drivers/modesetting/vblank.c

@@ -90,6 +90,39 @@ ms_crtc_on(xf86CrtcPtr crtc)
     return crtc->enabled && drmmode_crtc->dpms_mode == DPMSModeOn;
 }
 
+/*
+ * Return the first output which is connected to an active CRTC on this screen.
+ *
+ * RRFirstOutput() will return an output from a slave screen if it is primary,
+ * which is not the behavior that ms_covering_crtc() wants.
+ */
+
+static RROutputPtr ms_first_output(ScreenPtr pScreen)
+{
+    rrScrPriv(pScreen);
+    RROutputPtr output;
+    int i, j;
+
+    if (!pScrPriv)
+        return NULL;
+
+    if (pScrPriv->primaryOutput && pScrPriv->primaryOutput->crtc &&
+        (pScrPriv->primaryOutput->pScreen == pScreen)) {
+        return pScrPriv->primaryOutput;
+    }
+
+    for (i = 0; i < pScrPriv->numCrtcs; i++) {
+        RRCrtcPtr crtc = pScrPriv->crtcs[i];
+
+        for (j = 0; j < pScrPriv->numOutputs; j++) {
+            output = pScrPriv->outputs[j];
+            if (output->crtc == crtc)
+                return output;
+        }
+    }
+    return NULL;
+}
+
 /*
  * Return the crtc covering 'box'. If two crtcs cover a portion of
  * 'box', then prefer the crtc with greater coverage.
@@ -135,7 +168,7 @@ ms_covering_crtc(ScreenPtr pScreen, BoxPtr box, Bool screen_is_ms)
         ScreenPtr slave;
 
         if (dixPrivateKeyRegistered(rrPrivKey))
-            primary_output = RRFirstOutput(scrn->pScreen);
+            primary_output = ms_first_output(scrn->pScreen);
         if (!primary_output || !primary_output->crtc)
             return NULL;