drm: Share plane pixel format check code between legacy and atomic

Both the legacy and atomic helpers need to check whether a plane
supports a given pixel format. The code is currently duplicated, share
it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
[danvet: Slightly extend the docbook.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>