drm/docs: more leftovers from the big vtable documentation pile

Another pile of vfuncs from the old gpu.tmpl xml documentation that
I've forgotten to delete. I spotted a few more things to
clarify/extend in the new kerneldoc while going through this once
more.

v2: Spelling fixes (Thierry).

v3: More spelling fixes and use Thierry's proposal to clarify why
drivers need to validate modes both in ->mode_fixup and ->mode_valid.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Thierry Reding <treding@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>