Use buffer size directly instead of compensating for underscan

The ddx has been changed to use cropped buffers, so we must stop cropping size in kernel.
Obey buffer pitch

The exynos mixer's "span" should really use pitch - not width.
drm_crtc: Remove sanity checks from crtc setup

When using cropped underscan we need a frame buffer smaller than the crtc.  This patch defeats the checks that prevent that from happening.