drm/sun4i: tcon: fix check of tcon->panel null pointer

Since tcon->panel is a pointer returned by of_drm_find_panel() need to
check if it is not NULL, hence a valid pointer.
IS_ERR() instead checks return error values, not NULL pointers.

Substitute "if (!IS_ERR(tcon->panel))" with "if (tcon->panel)".

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181005215951.99003-1-giulio.benetti@micronovasrl.com