drm/tilcdc: fix leak & null ref in panel_connector_get_modes

If videomode_from_timings() returns true, the mode allocated with
drm_mode_create will be leaked.

Also, the return value of drm_mode_create() is never checked, and thus
could cause NULL deref.

Fix these two issues.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200429104234.18910-1-tomi.valkeinen@ti.com
Reviewed-by: Jyri Sarha <jsarha@ti.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>