drm/mediatek: mtk_dpi: remove invalid error message

Do not try to dereference dpi if it is NULL.
Since dpi can never be NULL when mtk_dpi_set_display_mode() is called,
remove the message.

Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>