drm/doc: Document color space handling

Again move it from the unmaintainable csv into DOC free-form overview
sections.

v2: Types Lionel&Sean spotted.

Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1474448370-32227-6-git-send-email-daniel.vetter@ffwll.ch