drm/exynos/hdmi: quirk for support mode timings conversion

MIXER in SoCs prior to Exynos5420 supports only 4 video modes:
720x480, 720x576, 1280x720, 1920x1080. Support for other modes
can be enabled by manipulating timings of HDMI. To do it
adjusted_mode should contain actual mode set on crtc.
With this patch it is possible to enable 1024x768 and 1280x1024
modes in MIXER.

Suggested-by: Daniel Drake <drake@endlessm.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Signed-off-by: Inki Dae <inki.dae@samsung.com>