drm/stm: check pitch and size calculations even if !CONFIG_MMU

In all cases we have to check pitch and size calculations to speed up
data transfer.

Fixes: 21f815bf

 ("drm/stm: drv: Improve data transfers")

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Tested-by: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Philippe Cornu <philippe.cornu@st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180131080508.14356-1-benjamin.gaignard@linaro.org