drm: fix use of freed memory in drm_mode_setcrtc

drm_mode_setcrtc() retries modesetting in case one of the functions it
calls returns -EDEADLK. connector_set, mode and fb are freed before
retrying, but they are not set to NULL. This can cause
drm_mode_setcrtc() to use those variables.

For example: On the first try __drm_mode_set_config_internal() returns
-EDEADLK. connector_set, mode and fb are freed. Next retry starts, and
drm_modeset_lock_all_ctx() returns -EDEADLK, and we jump to 'out'. The
code will happily try to release all three again.

This leads to crashes of different kinds, depending on the sequence the
EDEADLKs happen.

Fix this by setting the three variables to NULL at the start of the
retry loop.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180917110054.4053-1-tomi.valkeinen@ti.com