drm/stm: dsi: Remove unnecessary platform_get_resource() error check

devm_ioremap_resource() already checks if the resource is NULL, so
remove the unnecessary platform_get_resource() error check.

Cc: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1513602150-7542-5-git-send-email-festevam@gmail.com