drm/stm: ltdc: Cleanup signal polarity defines

The GCR_PCPOL/DEPOL/VSPOL/HSPOL defines are sufficient to
describe the HS, VS, DE & PC signal polarities.

Signed-off-by: Philippe CORNU <philippe.cornu@st.com>
Reviewed-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1500552357-29487-3-git-send-email-philippe.cornu@st.com