vgaarb: move the kerneldoc for vga_set_legacy_decoding to vgaarb.c

Kerneldoc comments should be at the implementation side, not in the
header just declaring the prototype.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210716061634.2446357-4-hch@lst.de


Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>