Fix loader detection of vbefb support on !amd64

On i386, after 6c7a932d0b8baaaee16eca0ba061bfa6e0e57bfd, the vbefb vt
driver was no longer detected by the loader, if any kernel module was
loaded after the kernel itself.

This was caused by the parse_vt_drv_set() function being called multiple
times, resetting the detection flag. (It was called multiple times,
becuase i386 .ko files are shared objects like the kernel proper, while
this is not the case on amd64.)

Fix this by skipping the set_vt_drv_set lookup if vbefb was already
detected.

Reviewed by: tsoome