modesetting: Internal storage of scanout pixmaps

modesetting relied on randr_crtc->scanout_pixmap being consistent with
calls to SetScanoutPixmap, which is very fragile and makes a lot of
assumptions about the caller's behavior.

For example, RRReplaceScanoutPixmap(), when dropping off with !size_fits,
will set randr_crtc->scanout_pixmap = NULL and then call SetScanoutPixmap.
Without this patch, drmmode_set_scanout_pixmap_(cpu/gpu) will think that
there is no scanout pixmap to tear down, because it's already been set to
NULL.

By keeping track of the scanout pixmap in its internal state, modesetting
can avoid these types of bugs and reduce constraints on calling
conventions.

v1: N/A
v2: N/A
v3: N/A
v4: N/A
v5: Initial commit
v6: Rebase onto ToT
v7: Unchanged

Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Alex Goins <agoins@nvidia.com>