drm: Exynos: Respect framebuffer pitch for FIMD/Mixer

When performing a modeset, use the framebuffer pitch value to set FIMD
IMG_SIZE and Mixer SPAN registers. These are both defined as pitch - the
distance between contiguous lines (bytes for FIMD, pixels for mixer).

Fixes display on Snow (1366x768).

Signed-off-by: Daniel Stone <daniels@collabora.com>