fix a bug about fb slot claimed conflict pci slot claimed

When the graphics card driver of the lower kernel version cannot support the graphics card, there will be no /dev/drm/card [0.. n] device node on the system, so the modestings driver will probe failed, and fbdev and VESA will probe successfully. So, fbslotclaimed will be assigned to true, and the value of pcislotclaimed will become non-zero after plus 1. When the program runs to the xf86postprobe() function, the error branch will be executed, and the fatalerror() function will be executed to print the error log: "cannot run in framebuffer mode. Please specify busids for all framebuffer devices", xorg will be forced to exit.