[Mesa-stable] fix for non gallivm draw context creation

[Jonathan Gray]

1e003b44e83dde3912ec48eb3df0e25802b5101e should be considered for
the 9.2 branch.

As seen in https://bugs.freedesktop.org/show_bug.cgi?id=66806 and
https://bugs.freedesktop.org/show_bug.cgi?id=67587 it fixes a hard
to diagnose problem and the fix doesn't seem to be present in the 9.2
branch yet despite what Roland says in the later bug report.