gallium: do not increase ref count of the new throttle fence

gallium: do not increase ref count of the new throttle fence

A new throttle fence was initialized to 1, and increased by 1
again when it's put in drawable->throttle_fence; the ref was
decreased by 1 when it's removed from drawable->throttle_fence,
and never reached to 0, caused leak.

Fixes: ff77bf5cbf7 ("gallium: simplify throttle implementation")
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1949
Signed-off-by: James Xiong <james.xiong@intel.com>
Reported-by: Florian Wesch <fw@info-beamer.com>
Reviewed-by: Jose Maria Casanova Crespo <jmcasanova@igalia.com>