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