drm/exynos: Add 1024x768/1280x800 HDMI resolutions

drm/exynos: Add 1024x768/1280x800 HDMI resolutions

This patch adds 1024x768 and 1280x800 HDMI resolutions by redistributing
vertical lines to the blanking period from the active area. The result
is that userspace is exposed resolutions which fit into the mixer's
scanning guidelines (720 lines), and the monitor thinks it's getting the
full monty. For example, 1024x768 looks like 1024x720 from userspace's
perspective, but 1024x768 from the monitor's perspective.

BUG=chromium:221411, chrome-os-partner:14238
TEST=Tested by hand with 1024x768

Change-Id: I6dcfe6260504e38aa604c5b19abc2bd716bbf729
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/48830
Reviewed-by: Mark Hayter <mdhayter@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>