vc4: Correct typo setting 'handled_qinst_cond'

vc4: Correct typo setting 'handled_qinst_cond'

Variable was previously always set to true. Accordingly, the later
assert() served no active purpose.

Found with GCC warning and code inspection:

mesa/src/gallium/drivers/vc4/vc4_qpu_emit.c: In function'vc4_generate_code':
mesa/src/gallium/drivers/vc4/vc4_qpu_emit.c:315:22: warning: variable 'handled_qinst_cond' set but not used [-Wunused-but-set-variable]
                 bool handled_qinst_cond = true;
                      ^

Signed-off-by: Rhys Kidd <rhyskidd@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>