Revert "drm/exynos: mixer: support for 800x600 resolution"

Revert "drm/exynos: mixer: support for 800x600 resolution"

This reverts commit ed244d6a805b3c11a72dbce6307ed752e7c15848.

The 800x600 resolution does not work, so let's not expose it as
it causes black screens, especially with projectors where this mode
is very common (we don't have 1024x768 so it falls back to 800x600).

BUG=chromium:225983
TEST=by hand: the 24 inch Dell monitor now picks 640x480 and works fine

Conflicts:
	drivers/gpu/drm/exynos/exynos_drm_hdmi.c

Change-Id: I0337a63767b8ed10f1879f8251f54ac6649b604b
Reviewed-on: https://gerrit.chromium.org/gerrit/47268
Tested-by: Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by: Stuart Abercrombie <sabercrombie@chromium.org>
Commit-Queue: Stéphane Marchesin <marcheu@chromium.org>