Add support for multiple modes to drm_output_propose_state. Currently we
intend to operate in three modes: planes-only (no renderer buffer,
client buffers in planes only), mixed-mode (promote client buffers to
planes where possible, falling back to the renderer where not), and
renderer-only (no plane usage at all).

We want to use the first (planes-only) mode where possible: it can avoid
us having to allocate buffers for the renderer, and it also gives us the
best chance of the optimal configuration, with no composition. In this
mode, we walk the scene looking at all views, trying to put them in
planes, and failing as soon as we find a view we cannot place in a
plane.

In the second mode, rather than failing, we assign those views which
cannot be on a plane to the renderer, and allow the renderer to
composite them.

In the third mode, planes are not usable, so everything but the cursor
goes to the renderer. We will use this when we cannot use the planes-only
mode (because some views cannot be placed in planes), but also cannot
use the 'mixed' mode because we have no renderer buffer yet. Since we
walk the scene graph from top to bottom, using atomic modesetting we
will determine if planes can be promoted in mixed mode by placing a
renderer buffer at the bottom of the scene, placing a cursor buffer if
applicable, then testing if we can add overlay planes to this mode.

Without a buffer from the renderer, we cannot do these tests, so we push
everything through the renderer and then switch to mixed mode on the
next repaint.

This patch implements the mixed and renderer-only modes (previously
differentiated only by the sprites_are_broken flag), with the
planes-only mode being left for a later patch.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>