drm/crtc: fix uninitialized variable use

Commit 3823119b ("drm/crtc: Fix uninit-value bug in
drm_mode_setcrtc") was supposed to fix use of an uninitialized variable,
but introduced another.

num_connectors is only initialized if crtc_req->count_connectors > 0,
but it's used regardless. Fix it.

Fixes: 3823119b ("drm/crtc: Fix uninit-value bug in drm_mode_setcrtc")
Cc: syzbot+4fad2e57beb6397ab2fc@syzkaller.appspotmail.com
Cc: Ziqi Zhao <astrajoan@yahoo.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231208131238.2924571-1-jani.nikula@intel.com