drm/exynos/mixer: pass actual mode on MIXER to encoder

drm/exynos/mixer: pass actual mode on MIXER to encoder

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 allow it MIXER must pass
actual video mode to HDMI, the proper way to do it is to modify
adjusted_mode property in crtc::mode_fixup callback. Adding such callback
allows also to simplify mixer_cfg_scan code - choosing mode is performed
already in crtc::mode_fixup. mode_fixup is also better place to check
interlace flag.

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>