drm/fb: handle tiled connectors better

We don't want tile 0,0 to artificially constrain the size of the legacy
fbdev device.  Instead when reducing fb_size to be the minimum of all
displays, only consider the rightmost and bottommost tiles.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Tested-by: Hai Li <hali@codeaurora.org>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>