mesa: set CurrentServerDispatch too when glBegin/End

mesa: set CurrentServerDispatch too when glBegin/End

When glthread not enabled, CurrentClientDispatch and CurrentServerDispatch
should be same. This does not cause problems before because OutsideBeginEnd
and BeginEnd have same BeginEnd entries, so when
CurrentServerDispatch==OutsideBeginEnd
CurrentClientDispatch==BeginEnd
will call into same BeginEnd _mesa_* functions.

But we'll add another dispatch table to replace BeginEnd when HW GL_SELECT
mode, so this needs to be fixed. Otherwise some function like _mesa_Rectf
which always call with CurrentServerDispatch will go into wrong entries.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Qiang Yu <yuq825@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15765>