drm/stm: ltdc: add mode_valid()

Add mode_valid() function to filter modes according to available
pll clock values and "preferred" modes. It is particularly
useful for hdmi modes that require precise pixel clocks.

Note that "preferred" modes are always accepted:
- this is important for panels because panel clock tolerances are
  bigger than hdmi ones and there is no reason to not accept them
  (the fps may vary a little but it is not a problem).
- the hdmi preferred mode will be accepted too, but userland will
  be able to use others hdmi "valid" modes if necessary.

Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Yannick Fertré <yannick.fertre@st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180417114026.8709-1-philippe.cornu@st.com