drm: Add crtc/encoder/bridge->mode_valid() callbacks

This adds a new callback to crtc, encoder and bridge helper functions
called mode_valid(). This callback shall be implemented if the
corresponding component has some sort of restriction in the modes
that can be displayed. A NULL callback implicates that the component
can display all the modes.

We also change the documentation so that the new and old callbacks
are correctly documented.

Only the callbacks were implemented to simplify review process,
following patches will make use of them.

Changes in v2 from Daniel:
- Update the warning about how modes aren't filtered in atomic_check -
  the heleprs help out a lot more now.
- Consistenly roll out that warning, crtc/encoder's atomic_check
  missed it.
- Sprinkle more links all over the place, so it's easier to see where
  this stuff is used and how the differen hooks are related.
- Note that ->mode_valid is optional everywhere.
- Explain why the connector's mode_valid is special and does _not_ get
  called in atomic_check.

v3: Document what can and cannot be checked in mode_valid a bit better
(Andrjez). Answer: Only allowed to look at the mode, nothing else.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Cc: Jose Abreu <joabreu@synopsys.com>
Cc: Carlos Palminha <palminha@synopsys.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Dave Airlie <airlied@linux.ie>
Cc: Andrzej Hajda <a.hajda@samsung.com>
Cc: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch> (v2)
Reviewed-by: Jose Abreu <joabreu@synopsys.com> (v2)
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170515093347.31098-1-daniel.vetter@ffwll.ch