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>