fix Xorg coredumps that have started happening recently.

the problem is that we get a SIGALRM while we're sleeping during a page fault
on a mapping of a GEM object, and since we're sleeping interruptibly,
the GEM operation fails with EINTR.  this error is returned all the way back
through uvm_fault() to the trap handler, which responds to that error
by delivering a SIGSEGV.

fix this by doing like the linux version of the GEM fault handler and converting
EINTR into success, which results in delivering the original signal and
retrying the fault.

sys/external/bsd/drm2/dist/drm/i915/i915_gem.c

@@ -1869,6 +1869,15 @@ out:
 	uvmfault_unlockall(ufi, ufi->entry->aref.ar_amap, uobj);
 	if (ret == -ERESTART)
 		uvm_wait("i915flt");
+
+	/*
+	 * Remap EINTR to success, so that we return to userland.
+	 * On the way out, we'll deliver the signal, and if the signal
+	 * is not fatal then the user code which faulted will most likely
+	 * fault again, and we'll come back here for another try.
+	 */
+	if (ret == -EINTR)
+		ret = 0;
 	/* XXX Deal with GPU hangs here...  */
 	intel_runtime_pm_put(dev_priv);
 	/* XXX errno Linux->NetBSD */