drm/exynos: mixer: add 2x scaling to mixer_graph_buffer

While the VP (video processor) supports arbitrary scaling
of its input, the mixer just supports a simple 2x (line
doubling) scaling. Expose this functionality and exit
early when an unsupported scaling configuration is
encountered.

This was tested with modetest's DRM plane test (from
the libdrm test suite) on an Odroid-X2 (Exynos4412).

v2: Put if- and return-statement on different lines.

Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Acked-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>