drm: todo: Avoid accidental crossreferences

RST uses underscores at the end of words to create crossreferences and
it will accidentally try to link to tinydrm_ and drm_fb_ targets from
the TODO, which is clearly not the intention in this context.

Use backslashes to escape the special meaning of the underscore.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170731124259.7495-1-thierry.reding@gmail.com