intel/dri: finish proper glthread

intel/dri: finish proper glthread

KWin was able to get NULL-context in the call
intelUnbindContext. But a call _mesa_glthread_finish
is not resistent to such case.
Case can be catched with steps:
	1. Create both glx and egl contexts
	2. Make glx as current
	3. Make egl as current
	4. Reset glx context
	5. Make egl as current

Solution adds proper finishing of glthread-context
(context will be taken from the requested dri-context
for unbinding, but not from the saved current context).

Piglit-test: https://gitlab.freedesktop.org/mesa/piglit/merge_requests/87

Cc: 19.1 19.2 <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110814
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111271
Fixes: dca36d5516d0 (i965: Implement threaded GL support)
Signed-off-by: Sergii Romantsov <sergii.romantsov@globallogic.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>