man: Remove description of DRM specific mode-options from weston.ini.man

The weston.ini.man describes the mode-formats that a user can specify
for selecting a video mode. The DRM specific examples are already
provided in weston-drm.man, so this inofrmation is redundant and can
be removed.

This patch removes the DRM specific mode option details from the
description of mode configuration in weston.ini.man.
A pointer to weston-drm.man is given, which has complete information
about the mode-format options supported by DRM backend.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>