drm/sti: fix GDP size to support up to UHD resolution

On stih407-410 chip family the GDP layers are able to support up to UHD
resolution (3840 x 2160).

Signed-off-by: Vincent Abriou <vincent.abriou@st.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Tested-by: Lee Jones <lee.jones@linaro.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1490280292-30466-1-git-send-email-vincent.abriou@st.com