drm/atomic: better doc for implicit vs explicit fencing

Note that a pile of drivers don't seem to take implicit fencing into
account, or at least don't call drm_atoimc_set_fence_for_plane().
Cc'ing relevant people, or at least some. Some drivers also look like
they don't disable implicit fencing (e.g. amdgpu) because the explicit
fences and implicit fences are handled by entirely independent code
paths.

I also wonder whether we shouldn't just make the recommended helpers
the default ones, since a lot of drivers don't bother to handle the
implicit fences at all it seems. The helpers won't blow up even for
non-GEM drivers or GEM drivers which don't fill out the gem bo
pointers in struct drm_framebuffer.

v2: Comments from Eric.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Sinclair Yeh <syeh@vmware.com>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Cc: Gustavo Padovan <gustavo@padovan.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180405154449.23038-7-daniel.vetter@ffwll.ch