drm: simple_kms_helper: Fix .mode_valid() documentation

This fixes up the .mode_valid() vtable entry documentation
by copyediting the documentation from the .mode_valid()
documentation in the drm_modeset_helper_vtables.h file.

Fixes: 40275dc4 ("drm: simple_kms_helper: Add mode_valid() callback support")
Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180227101109.6088-1-linus.walleij@linaro.org