drm/stm: ltdc: add devm_reset_control & platform_get_ressource

Use devm_reset_control_get_exclusive to avoid resource leakage (based
on patch "Convert drivers to explicit reset API" from Philipp Zabel).

Also use platform_get_resource, which is more usual and
consistent with platform_get_irq called later.

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: Philippe CORNU <philippe.cornu@st.com>
Reviewed-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1500552357-29487-6-git-send-email-philippe.cornu@st.com