drm/doc: Document feature merge deadlines

The discussion pretty much concluded without objections, let's
document what we agreed on.

Cc'ing linux-doc for the new tag in Documentation/process/index.rst.

Acked-by: Dave Airlie <airlied@gmail.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Cc: Dave Airlie <airlied@gmail.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Alex Deucher <alexdeucher@gmail.com>
Cc: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170321155228.30287-1-daniel.vetter@ffwll.ch