drm: Move more framebuffer doc from docbook to kerneldoc

I missed a few paragraphs in the docbook that need to be pulled into
the fbdev vfunc docs.

v2: Spelling fixes from Thierry.

Cc: Thierry Reding <treding@nvidia.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449564561-3896-3-git-send-email-daniel.vetter@ffwll.ch


Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>