drm/i915: Remove unnecessary null check in execlists_context_unqueue

commit 53292cdb ("drm/i915: Workaround
to avoid lite restore with HEAD==TAIL") added a check for req0 != null
which is unnecessary.

The only way req0 could be null is if the list was empty, and this is
already addressed at the beginning of execlists_context_unqueue().

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>