drm/meson: Add support for DMT modes on HDMI

This patch adds support for DMT display modes over HDMI.
The modes timings configurations are from the Amlogic Vendor linux tree
and tested over multiples monitors.
Previously only a selected number of CEA modes were supported.

Only these following modes are supported with these changes:
- 640x480@60Hz
- 800x600@60Hz
- 1024x768@60Hz
- 1152x864@75Hz
- 1280x1024@60Hz
- 1600x1200@60Hz
- 1920x1080@60Hz

The associated code to handle the clock rates is also added.

Acked-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1520935670-14187-1-git-send-email-narmstrong@baylibre.com

drivers/gpu/drm/meson/meson_venc.h

@@ -58,6 +58,7 @@ struct meson_cvbs_enci_mode {
 };
 
 /* HDMI Clock parameters */
+bool meson_venc_hdmi_supported_mode(const struct drm_display_mode *mode);
 bool meson_venc_hdmi_supported_vic(int vic);
 bool meson_venc_hdmi_venc_repeat(int vic);