Read out the current plane configuration at init time into a new
plane_config structure.  This allows us to track any existing
framebuffers attached to the plane and potentially re-use them in our
fbdev code for a smooth handoff.

v2: update for new pitch_for_width function (Jesse)
    comment how get_plane_config works with shared fbs (Jesse)
v3: s/ARGB/XRGB (Ville)
    use pipesrc width/height (Ville)
    fix fourcc comment (Bob)
    use drm_format_plane_cpp (Ville)
v4: use fb for tracking fb data object (Ville)
v5: fix up gen2 pitch limits (Ville)
v6: read out stride as well (Daniel)
v7: split out init ordering changes (Daniel)
    don't fetch config if !CONFIG_FB
v8: use proper height in get_plane_config (Chris)
v9: fix CONFIG_FB check for modular configs (Jani)
v10: add comment about stolen allocation stomping
v11: drop hw state readout hunk (Daniel)
v12: handle tiled BIOS fbs (Kristian)
     pull out common bits (Jesse)
v13: move fb obj alloc out to _init
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>